diff --git a/src/idl_gen_csharp.cpp b/src/idl_gen_csharp.cpp
index 05fe04ad7c7..f23d9f07fb6 100644
--- a/src/idl_gen_csharp.cpp
+++ b/src/idl_gen_csharp.cpp
@@ -309,17 +309,17 @@ class CSharpGenerator : public BaseGenerator {
   // would be cast down to int before being put onto the buffer. In C#, one cast
   // directly cast an Enum to its underlying type, which is essential before
   // putting it onto the buffer.
-  std::string SourceCast(const Type &type) const {
+  std::string SourceCast(const Type &type, const bool isOptional=false) const {
     if (IsSeries(type)) {
       return SourceCast(type.VectorType());
     } else {
-      if (IsEnum(type)) return "(" + GenTypeBasic(type, false) + ")";
+      if (IsEnum(type)) return "(" + GenTypeBasic(type, false) + (isOptional ? "?": "") + ")";
     }
     return "";
   }
 
-  std::string SourceCastBasic(const Type &type) const {
-    return IsScalar(type.base_type) ? SourceCast(type) : "";
+  std::string SourceCastBasic(const Type &type, const bool isOptional) const {
+    return IsScalar(type.base_type) ? SourceCast(type, isOptional) : "";
   }
 
   std::string GenEnumDefaultValue(const FieldDef &field) const {
@@ -1191,7 +1191,7 @@ class CSharpGenerator : public BaseGenerator {
         code += " " + EscapeKeyword(argname) + ") { builder.Add";
         code += GenMethod(field.value.type) + "(";
         code += NumToString(it - struct_def.fields.vec.begin()) + ", ";
-        code += SourceCastBasic(field.value.type);
+        code += SourceCastBasic(field.value.type, field.IsScalarOptional());
         code += EscapeKeyword(argname);
         if (!IsScalar(field.value.type.base_type) &&
             field.value.type.base_type != BASE_TYPE_UNION) {
@@ -1225,7 +1225,9 @@ class CSharpGenerator : public BaseGenerator {
             code += "Add";
             code += GenMethod(vector_type);
             code += "(";
-            code += SourceCastBasic(vector_type);
+            // At the moment there is no support of the type Vector with optional enum, 
+            // e.g. if we have enum type SomeEnum there is no way to define `SomeEmum?[] enums` in FlatBuffer schema, so isOptional = false
+            code += SourceCastBasic(vector_type, false);
             code += "data[i]";
             if (vector_type.base_type == BASE_TYPE_STRUCT ||
                 IsString(vector_type))
diff --git a/tests/FlatBuffers.Test/FlatBuffersExampleTests.cs b/tests/FlatBuffers.Test/FlatBuffersExampleTests.cs
index 83cb35c2e13..79a213c3880 100644
--- a/tests/FlatBuffers.Test/FlatBuffersExampleTests.cs
+++ b/tests/FlatBuffers.Test/FlatBuffersExampleTests.cs
@@ -1139,5 +1139,18 @@ public void TestKeywordEscaping() {
             Assert.AreEqual(keywordsInTable.Is, KeywordTest.ABC.@stackalloc);
             Assert.AreEqual(keywordsInTable.Private, KeywordTest.@public.NONE);
         }
+
+
+        [FlatBuffersTestMethod]
+        public void AddOptionalEnum_WhenPassNull_ShouldWorkProperly() {
+          var fbb = new FlatBufferBuilder(1);
+          ScalarStuff.StartScalarStuff(fbb);
+          ScalarStuff.AddMaybeEnum(fbb, null);
+          var offset = ScalarStuff.EndScalarStuff(fbb);
+          ScalarStuff.FinishScalarStuffBuffer(fbb, offset);
+          
+          ScalarStuff scalarStuff = ScalarStuff.GetRootAsScalarStuff(fbb.DataBuffer);
+          Assert.AreEqual(null, scalarStuff.MaybeEnum);
+        }
     }
 }
diff --git a/tests/optional_scalars/ScalarStuff.cs b/tests/optional_scalars/ScalarStuff.cs
index f7ad73a8004..e5498383082 100644
--- a/tests/optional_scalars/ScalarStuff.cs
+++ b/tests/optional_scalars/ScalarStuff.cs
@@ -169,7 +169,7 @@ public struct ScalarStuff : IFlatbufferObject
   public static void AddMaybeBool(FlatBufferBuilder builder, bool? maybeBool) { builder.AddBool(31, maybeBool); }
   public static void AddDefaultBool(FlatBufferBuilder builder, bool defaultBool) { builder.AddBool(32, defaultBool, true); }
   public static void AddJustEnum(FlatBufferBuilder builder, optional_scalars.OptionalByte justEnum) { builder.AddSbyte(33, (sbyte)justEnum, 0); }
-  public static void AddMaybeEnum(FlatBufferBuilder builder, optional_scalars.OptionalByte? maybeEnum) { builder.AddSbyte(34, (sbyte)maybeEnum); }
+  public static void AddMaybeEnum(FlatBufferBuilder builder, optional_scalars.OptionalByte? maybeEnum) { builder.AddSbyte(34, (sbyte?)maybeEnum); }
   public static void AddDefaultEnum(FlatBufferBuilder builder, optional_scalars.OptionalByte defaultEnum) { builder.AddSbyte(35, (sbyte)defaultEnum, 1); }
   public static Offset<optional_scalars.ScalarStuff> EndScalarStuff(FlatBufferBuilder builder) {
     int o = builder.EndTable();