diff --git a/MarkMpn.Sql4Cds.Engine.Tests/ExecutionPlanTests.cs b/MarkMpn.Sql4Cds.Engine.Tests/ExecutionPlanTests.cs
index 854836db..1456b311 100644
--- a/MarkMpn.Sql4Cds.Engine.Tests/ExecutionPlanTests.cs
+++ b/MarkMpn.Sql4Cds.Engine.Tests/ExecutionPlanTests.cs
@@ -3714,7 +3714,7 @@ GROUP BY firstname
                 },
             };
             
-            var result = select.Execute(new NodeExecutionContext(_localDataSources, this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var result = select.Execute(new NodeExecutionContext(_localDataSources, this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(result);
 
@@ -4192,7 +4192,7 @@ DECLARE @test int
             Assert.AreEqual(1, selectConstantScan.Values.Count);
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
@@ -4246,7 +4246,7 @@ public void SetVariableInDeclaration()
             Assert.AreEqual(1, selectConstantScan.Values.Count);
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
@@ -4304,7 +4304,7 @@ DECLARE @test varchar(3)
             var plans = planBuilder.Build(query, null, out _);
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
@@ -4338,7 +4338,7 @@ DECLARE @test varchar(3)
             var plans = planBuilder.Build(query, null, out _);
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
@@ -4430,7 +4430,7 @@ DECLARE @test varchar(3)
             };
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
@@ -4464,7 +4464,7 @@ DECLARE @test varchar
             var plans = planBuilder.Build(query, null, out _);
 
             var parameterTypes = new Dictionary<string, DataTypeReference>();
-            var parameterValues = new Dictionary<string, object>();
+            var parameterValues = new Dictionary<string, INullable>();
 
             foreach (var plan in plans)
             {
diff --git a/MarkMpn.Sql4Cds.Engine.Tests/Sql2FetchXmlTests.cs b/MarkMpn.Sql4Cds.Engine.Tests/Sql2FetchXmlTests.cs
index 8e601557..04c8c63f 100644
--- a/MarkMpn.Sql4Cds.Engine.Tests/Sql2FetchXmlTests.cs
+++ b/MarkMpn.Sql4Cds.Engine.Tests/Sql2FetchXmlTests.cs
@@ -702,7 +702,7 @@ public void SelectArithmetic()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -752,7 +752,7 @@ public void WhereComparingTwoFields()
                     }
                 };
 
-                var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+                var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
                 var dataTable = new DataTable();
                 dataTable.Load(dataReader);
 
@@ -797,7 +797,7 @@ public void WhereComparingExpression()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -838,7 +838,7 @@ public void BackToFrontLikeExpression()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -873,7 +873,7 @@ public void UpdateFieldToField()
                 }
             };
 
-            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), out _, out _);
+            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), out _, out _);
 
             Assert.AreEqual("Carrington", _context.Data["contact"][guid]["firstname"]);
         }
@@ -905,7 +905,7 @@ public void UpdateFieldToExpression()
                 }
             };
 
-            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), out _, out _);
+            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), out _, out _);
 
             Assert.AreEqual("Hello Carrington", _context.Data["contact"][guid]["firstname"]);
         }
@@ -941,7 +941,7 @@ public void UpdateReplace()
                 }
             };
 
-            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), out _, out _);
+            ((UpdateNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), out _, out _);
 
             Assert.AreEqual("--CDS--", _context.Data["contact"][guid]["firstname"]);
         }
@@ -972,7 +972,7 @@ public void StringFunctions()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1012,7 +1012,7 @@ public void SelectExpression()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1062,7 +1062,7 @@ public void SelectExpressionNullValues()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1107,7 +1107,7 @@ public void OrderByExpression()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1152,7 +1152,7 @@ public void OrderByAliasedField()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1196,7 +1196,7 @@ public void OrderByCalculatedField()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1240,7 +1240,7 @@ public void OrderByCalculatedFieldByIndex()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1282,7 +1282,7 @@ public void DateCalculations()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1370,7 +1370,7 @@ public void CustomFilterAggregateHavingProjectionSortAndTop()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1433,7 +1433,7 @@ public void FilterCaseInsensitive()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1490,7 +1490,7 @@ public void GroupCaseInsensitive()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1543,7 +1543,7 @@ public void AggregateExpressionsWithoutGrouping()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1598,7 +1598,7 @@ public void AggregateQueryProducesAlternative()
                 }
             };
 
-            var dataReader = alternativeQuery.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = alternativeQuery.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -1654,7 +1654,7 @@ public void GuidEntityReferenceInequality()
                     }
                 };
 
-                var dataReader = select.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+                var dataReader = select.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
                 var dataTable = new DataTable();
                 dataTable.Load(dataReader);
 
@@ -1706,7 +1706,7 @@ public void UpdateGuidToEntityReference()
                 }
             };
 
-            update.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), out _, out _);
+            update.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), out _, out _);
 
             Assert.AreEqual(new EntityReference("contact", contact1), _context.Data["account"][account1].GetAttributeValue<EntityReference>("primarycontactid"));
             Assert.AreEqual(new EntityReference("contact", contact2), _context.Data["account"][account2].GetAttributeValue<EntityReference>("primarycontactid"));
@@ -1981,7 +1981,7 @@ public void ImplicitTypeConversion()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2014,7 +2014,7 @@ public void ImplicitTypeConversionComparison()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2038,7 +2038,7 @@ public void GlobalOptionSet()
             Assert.AreEqual("globaloptionset.name = 'test'", filterNode.Filter.ToSql());
             var optionsetNode = (GlobalOptionSetQueryNode)filterNode.Source;
 
-            var dataReader = selectNode.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = selectNode.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2059,7 +2059,7 @@ public void EntityDetails()
             var sortNode = (SortNode)selectNode.Source;
             var metadataNode = (MetadataQueryNode)sortNode.Source;
 
-            var dataReader = selectNode.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = selectNode.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2077,7 +2077,7 @@ public void AttributeDetails()
             var planBuilder = new ExecutionPlanBuilder(_localDataSources.Values, this);
             var queries = planBuilder.Build(query, null, out _);
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2135,7 +2135,7 @@ public void OptionSetNameSelect()
 
             CollectionAssert.AreEqual(new[] { "new_optionsetvalue", "new_optionsetvaluename" }, select.ColumnSet.Select(c => c.OutputColumn).ToList());
 
-            var dataReader = select.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = select.Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2459,7 +2459,7 @@ public void CharIndex()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2489,7 +2489,7 @@ public void CastDateTimeToDate()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
@@ -2527,7 +2527,7 @@ public void GroupByPrimaryFunction()
                 }
             };
 
-            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, object>(), null), CommandBehavior.Default);
+            var dataReader = ((SelectNode)queries[0]).Execute(new NodeExecutionContext(GetDataSources(_context), this, new Dictionary<string, DataTypeReference>(), new Dictionary<string, INullable>(), null), CommandBehavior.Default);
             var dataTable = new DataTable();
             dataTable.Load(dataReader);
 
diff --git a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsConnection.cs b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsConnection.cs
index 15578c28..167f66ac 100644
--- a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsConnection.cs
+++ b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsConnection.cs
@@ -29,7 +29,7 @@ public class Sql4CdsConnection : DbConnection
         private readonly IDictionary<string, DataSource> _dataSources;
         private readonly ChangeDatabaseOptionsWrapper _options;
         private readonly Dictionary<string, DataTypeReference> _globalVariableTypes;
-        private readonly Dictionary<string, object> _globalVariableValues;
+        private readonly Dictionary<string, INullable> _globalVariableValues;
         private readonly TelemetryClient _ai;
 
         /// <summary>
@@ -74,7 +74,7 @@ public Sql4CdsConnection(IDictionary<string, DataSource> dataSources)
                 ["@@VERSION"] = DataTypeHelpers.NVarChar(Int32.MaxValue, _dataSources[_options.PrimaryDataSource].DefaultCollation, CollationLabel.CoercibleDefault),
                 ["@@ERROR"] = DataTypeHelpers.Int,
             };
-            _globalVariableValues = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase)
+            _globalVariableValues = new Dictionary<string, INullable>(StringComparer.OrdinalIgnoreCase)
             {
                 ["@@IDENTITY"] = SqlEntityReference.Null,
                 ["@@ROWCOUNT"] = (SqlInt32)0,
@@ -291,7 +291,7 @@ public ColumnOrdering ColumnOrdering
 
         internal Dictionary<string, DataTypeReference> GlobalVariableTypes => _globalVariableTypes;
 
-        internal Dictionary<string, object> GlobalVariableValues => _globalVariableValues;
+        internal Dictionary<string, INullable> GlobalVariableValues => _globalVariableValues;
 
         internal TelemetryClient TelemetryClient => _ai;
 
diff --git a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsDataReader.cs b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsDataReader.cs
index 3354c182..a88c27d0 100644
--- a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsDataReader.cs
+++ b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsDataReader.cs
@@ -22,7 +22,7 @@ class Sql4CdsDataReader : DbDataReader
         private readonly IQueryExecutionOptions _options;
         private readonly CommandBehavior _behavior;
         private readonly Dictionary<string, DataTypeReference> _parameterTypes;
-        private readonly Dictionary<string, object> _parameterValues;
+        private readonly Dictionary<string, INullable> _parameterValues;
         private Dictionary<string, int> _labelIndexes;
         private int _recordsAffected;
         private int _instructionPointer;
@@ -57,7 +57,7 @@ public Sql4CdsDataReader(Sql4CdsCommand command, IQueryExecutionOptions options,
                 Close();
         }
 
-        internal Dictionary<string, object> ParameterValues => _parameterValues;
+        internal Dictionary<string, INullable> ParameterValues => _parameterValues;
 
         private Dictionary<string, int> LabelIndexes
         {
@@ -75,7 +75,7 @@ private Dictionary<string, int> LabelIndexes
             }
         }
 
-        private bool ExecuteWithExceptionHandling(Dictionary<string, DataTypeReference> parameterTypes, Dictionary<string, object> parameterValues)
+        private bool ExecuteWithExceptionHandling(Dictionary<string, DataTypeReference> parameterTypes, Dictionary<string, INullable> parameterValues)
         {
             while (true)
             {
@@ -109,7 +109,7 @@ private bool ExecuteWithExceptionHandling(Dictionary<string, DataTypeReference>
             }
         }
 
-        private bool Execute(Dictionary<string, DataTypeReference> parameterTypes, Dictionary<string, object> parameterValues)
+        private bool Execute(Dictionary<string, DataTypeReference> parameterTypes, Dictionary<string, INullable> parameterValues)
         {
             IRootExecutionPlanNode logNode = null;
             var context = new NodeExecutionContext(_connection.DataSources, _options, parameterTypes, parameterValues, msg => _connection.OnInfoMessage(logNode, msg));
diff --git a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsParameterCollection.cs b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsParameterCollection.cs
index c5ea90eb..705df781 100644
--- a/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsParameterCollection.cs
+++ b/MarkMpn.Sql4Cds.Engine/Ado/Sql4CdsParameterCollection.cs
@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Data.Common;
+using System.Data.SqlTypes;
 using System.Linq;
 using System.Text;
 using Microsoft.SqlServer.TransactSql.ScriptDom;
@@ -50,11 +51,11 @@ internal Dictionary<string, DataTypeReference> GetParameterTypes()
                 .ToDictionary(param => param.FullParameterName, param => param.GetDataType(), StringComparer.OrdinalIgnoreCase);
         }
 
-        internal Dictionary<string, object> GetParameterValues()
+        internal Dictionary<string, INullable> GetParameterValues()
         {
             return _parameters
                 .Cast<Sql4CdsParameter>()
-                .ToDictionary(param => param.FullParameterName, param => (object) param.GetValue(), StringComparer.OrdinalIgnoreCase);
+                .ToDictionary(param => param.FullParameterName, param => param.GetValue(), StringComparer.OrdinalIgnoreCase);
         }
 
         public override bool Contains(string value)
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AssignVariablesNode.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AssignVariablesNode.cs
index 7f65da59..af59ef61 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AssignVariablesNode.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AssignVariablesNode.cs
@@ -54,7 +54,7 @@ public override void Execute(NodeExecutionContext context, out int recordsAffect
                 foreach (var entity in entities)
                 {
                     foreach (var variable in Variables)
-                        context.ParameterValues[variable.VariableName] = valueAccessors[variable.VariableName](entity);
+                        context.ParameterValues[variable.VariableName] = (INullable)valueAccessors[variable.VariableName](entity);
 
                     count++;
                 }
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/InsertNode.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/InsertNode.cs
index c24ae1aa..1c15ba0a 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/InsertNode.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/InsertNode.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Data.SqlTypes;
 using System.Linq;
 using System.ServiceModel;
 using System.Threading;
@@ -292,7 +293,7 @@ protected override bool FilterErrors(NodeExecutionContext context, OrganizationR
             return true;
         }
 
-        private void SetIdentity(OrganizationResponse response, IDictionary<string, object> parameterValues)
+        private void SetIdentity(OrganizationResponse response, IDictionary<string, INullable> parameterValues)
         {
             if (response is CreateResponse create)
                 parameterValues["@@IDENTITY"] = new SqlEntityReference(DataSource, LogicalName, create.id);
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/NestedLoopNode.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/NestedLoopNode.cs
index de391dc0..069d9c91 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/NestedLoopNode.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/NestedLoopNode.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Data.SqlTypes;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
@@ -71,14 +72,14 @@ protected override IEnumerable<Entity> ExecuteInternal(NodeExecutionContext cont
                 if (OuterReferences != null)
                 {
                     if (innerParameters == null)
-                        innerParameters = new Dictionary<string, object>();
+                        innerParameters = new Dictionary<string, INullable>();
                     else
-                        innerParameters = new Dictionary<string, object>(innerParameters);
+                        innerParameters = new Dictionary<string, INullable>(innerParameters);
 
                     foreach (var kvp in OuterReferences)
                     {
                         left.Attributes.TryGetValue(kvp.Key, out var outerValue);
-                        innerParameters[kvp.Value] = outerValue;
+                        innerParameters[kvp.Value] = (INullable)outerValue;
                     }
                 }
 
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/PartitionedAggregateNode.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/PartitionedAggregateNode.cs
index 9041e45d..bbc31602 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/PartitionedAggregateNode.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/PartitionedAggregateNode.cs
@@ -187,7 +187,7 @@ protected override IEnumerable<Entity> ExecuteInternal(NodeExecutionContext cont
                             Parent = this
                         };
 
-                        var partitionParameterValues = new Dictionary<string, object>
+                        var partitionParameterValues = new Dictionary<string, INullable>
                         {
                             ["@PartitionStart"] = minKey,
                             ["@PartitionEnd"] = maxKey
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/SelectDataReader.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/SelectDataReader.cs
index ff5b55ab..1ceef794 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/SelectDataReader.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/SelectDataReader.cs
@@ -17,12 +17,12 @@ class SelectDataReader : DbDataReader
         private readonly IDisposable _timer;
         private readonly INodeSchema _schema;
         private readonly IEnumerator<Entity> _source;
-        private readonly IDictionary<string, object> _parameterValues;
+        private readonly IDictionary<string, INullable> _parameterValues;
         private Entity _row;
         private bool _closed;
         private int _rowCount;
 
-        public SelectDataReader(List<SelectColumn> columnSet, IDisposable timer, INodeSchema schema, IEnumerable<Entity> source, IDictionary<string, object> parameterValues)
+        public SelectDataReader(List<SelectColumn> columnSet, IDisposable timer, INodeSchema schema, IEnumerable<Entity> source, IDictionary<string, INullable> parameterValues)
         {
             _columnSet = columnSet;
             _timer = timer;
diff --git a/MarkMpn.Sql4Cds.Engine/NodeContext.cs b/MarkMpn.Sql4Cds.Engine/NodeContext.cs
index 22034f62..68b6c98d 100644
--- a/MarkMpn.Sql4Cds.Engine/NodeContext.cs
+++ b/MarkMpn.Sql4Cds.Engine/NodeContext.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Data.SqlTypes;
 using System.Text;
 using MarkMpn.Sql4Cds.Engine.ExecutionPlan;
 using Microsoft.SqlServer.TransactSql.ScriptDom;
@@ -105,7 +106,7 @@ public NodeExecutionContext(
             IDictionary<string, DataSource> dataSources,
             IQueryExecutionOptions options,
             IDictionary<string, DataTypeReference> parameterTypes,
-            IDictionary<string, object> parameterValues,
+            IDictionary<string, INullable> parameterValues,
             Action<Sql4CdsError> log)
             : base(dataSources, options, parameterTypes, log)
         {
@@ -115,7 +116,7 @@ public NodeExecutionContext(
         /// <summary>
         /// Returns the current value of each parameter
         /// </summary>
-        public IDictionary<string, object> ParameterValues { get; }
+        public IDictionary<string, INullable> ParameterValues { get; }
 
         public Sql4CdsError Error { get; set; }
     }
@@ -191,7 +192,7 @@ public ExpressionExecutionContext(
             IDictionary<string, DataSource> dataSources,
             IQueryExecutionOptions options,
             IDictionary<string, DataTypeReference> parameterTypes,
-            IDictionary<string, object> parameterValues,
+            IDictionary<string, INullable> parameterValues,
             Action<Sql4CdsError> log,
             Entity entity)
             : base(dataSources, options, parameterTypes, parameterValues, log)