diff --git a/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessage.java b/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessage.java
index f8ab491686..ccda28cf57 100644
--- a/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessage.java
+++ b/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessage.java
@@ -20,8 +20,9 @@
 import com.google.protobuf.DynamicMessage;
 import com.google.protobuf.Message;
 import java.util.HashMap;
-import java.util.Map;
 import java.util.List;
+import java.util.Set;
+import java.util.TreeSet;
 import org.json.JSONArray;
 import org.json.JSONException;
 import org.json.JSONObject;
@@ -40,9 +41,9 @@ public class JsonToProtoMessage {
   public static DynamicMessage convertJsonToProtoMessage(
       Descriptor protoSchema, JSONObject json, boolean allowUnknownFields)
       throws IllegalArgumentException {
-      if (json.length() == 0) {
-        throw new IllegalArgumentException("JSONObject is empty.");
-      }
+    if (json.length() == 0) {
+      throw new IllegalArgumentException("JSONObject is empty.");
+    }
     return convertJsonToProtoMessageImpl(protoSchema, json, "root", true, allowUnknownFields);
   }
 
@@ -66,32 +67,37 @@ private static DynamicMessage convertJsonToProtoMessageImpl(
 
     DynamicMessage.Builder protoMsg = DynamicMessage.newBuilder(protoSchema);
     List<FieldDescriptor> protoFields = protoSchema.getFields();
-    HashMap<String, FieldDescriptor> protoLowercaseNameToString = new HashMap<String, FieldDescriptor>();
+
+    Set<String> protoFieldNames = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
     for (FieldDescriptor field : protoFields) {
-      protoLowercaseNameToString.put(field.getName().toLowerCase(), field);
+      protoFieldNames.add(field.getName());
     }
 
-    HashMap<String, String> jsonLowercaseNameToActualName = new HashMap<String, String>();
-    String[] actualNames = JSONObject.getNames(json);
-    for (int i = 0; i < actualNames.length; i++) {
-      jsonLowercaseNameToActualName.put(actualNames[i].toLowerCase(), actualNames[i]);
+    HashMap<String, String> jsonLowercaseNameToName = new HashMap<String, String>();
+    String[] jsonNames = JSONObject.getNames(json);
+    for (int i = 0; i < jsonNames.length; i++) {
+      jsonLowercaseNameToName.put(jsonNames[i].toLowerCase(), jsonNames[i]);
     }
+
     if (!allowUnknownFields) {
-      for (String jsonLowercaseField : jsonLowercaseNameToActualName.keySet()) {
-        if (!protoLowercaseNameToString.containsKey(jsonLowercaseField)) {
+      for (int i = 0; i < jsonNames.length; i++) {
+        if (!protoFieldNames.contains(jsonNames[i])) {
           throw new IllegalArgumentException(
-              "JSONObject has fields unknown to BigQuery: " + jsonScope + "." + jsonLowercaseNameToActualName.get(jsonLowercaseField) + ". Set allowUnknownFields to True to allow unknown fields.");
+              "JSONObject has fields unknown to BigQuery: "
+                  + jsonScope
+                  + "."
+                  + jsonNames[i]
+                  + ". Set allowUnknownFields to True to allow unknown fields.");
         }
       }
     }
 
     int matchedFields = 0;
-    for (Map.Entry<String, FieldDescriptor> protoEntry : protoLowercaseNameToString.entrySet()) {
-      String lowercaseFieldName = protoEntry.getKey();
-      FieldDescriptor field = protoEntry.getValue();
+    for (FieldDescriptor field : protoFields) {
+      String lowercaseFieldName = field.getName().toLowerCase();
       String currentScope = jsonScope + "." + field.getName();
 
-      if (!jsonLowercaseNameToActualName.containsKey(lowercaseFieldName)) {
+      if (!jsonLowercaseNameToName.containsKey(lowercaseFieldName)) {
         if (field.isRequired()) {
           throw new IllegalArgumentException(
               "JSONObject does not have the required field " + currentScope + ".");
@@ -105,7 +111,7 @@ private static DynamicMessage convertJsonToProtoMessageImpl(
             protoMsg,
             field,
             json,
-            jsonLowercaseNameToActualName.get(lowercaseFieldName),
+            jsonLowercaseNameToName.get(lowercaseFieldName),
             currentScope,
             allowUnknownFields);
       } else {
@@ -113,7 +119,7 @@ private static DynamicMessage convertJsonToProtoMessageImpl(
             protoMsg,
             field,
             json,
-            jsonLowercaseNameToActualName.get(lowercaseFieldName),
+            jsonLowercaseNameToName.get(lowercaseFieldName),
             currentScope,
             allowUnknownFields);
       }
@@ -283,12 +289,7 @@ private static void fillRepeatedField(
             protoMsg.addRepeatedField(fieldDescriptor, new Long((Long) val));
           } else {
             throw new IllegalArgumentException(
-                "JSONObject does not have a int64 field at "
-                    + currentScope
-                    + "["
-                    + i
-                    + "]"
-                    + ".");
+                "JSONObject does not have a int64 field at " + currentScope + "[" + i + "]" + ".");
           }
         }
         break;
@@ -299,12 +300,7 @@ private static void fillRepeatedField(
             protoMsg.addRepeatedField(fieldDescriptor, new Integer((Integer) val));
           } else {
             throw new IllegalArgumentException(
-                "JSONObject does not have a int32 field at "
-                    + currentScope
-                    + "["
-                    + i
-                    + "]"
-                    + ".");
+                "JSONObject does not have a int32 field at " + currentScope + "[" + i + "]" + ".");
           }
         }
         break;
@@ -327,12 +323,7 @@ private static void fillRepeatedField(
             protoMsg.addRepeatedField(fieldDescriptor, new Double((float) val));
           } else {
             throw new IllegalArgumentException(
-                "JSONObject does not have a double field at "
-                    + currentScope
-                    + "["
-                    + i
-                    + "]"
-                    + ".");
+                "JSONObject does not have a double field at " + currentScope + "[" + i + "]" + ".");
           }
         }
         break;
diff --git a/google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessageTest.java b/google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessageTest.java
index 4676764aed..4a711b0293 100644
--- a/google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessageTest.java
+++ b/google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1alpha2/JsonToProtoMessageTest.java
@@ -108,13 +108,7 @@ public class JsonToProtoMessageTest {
                   (double) Float.MIN_VALUE
                 })),
     new JSONObject()
-        .put(
-            "test_repeated",
-            new JSONArray(
-                new Float[] {
-                  Float.MAX_VALUE,
-                  Float.MIN_VALUE
-                })),
+        .put("test_repeated", new JSONArray(new Float[] {Float.MAX_VALUE, Float.MIN_VALUE})),
     new JSONObject().put("test_repeated", new JSONArray(new Boolean[] {true, false})),
     new JSONObject().put("test_repeated", new JSONArray(new String[] {"hello", "test"})),
     new JSONObject()
@@ -329,7 +323,8 @@ public void testAllRepeatedTypesWithLimits() throws Exception {
                   + " field at root.test_repeated[0].");
         }
       }
-      if (entry.getKey() == RepeatedInt64.getDescriptor() || entry.getKey() == RepeatedDouble.getDescriptor()) {
+      if (entry.getKey() == RepeatedInt64.getDescriptor()
+          || entry.getKey() == RepeatedDouble.getDescriptor()) {
         assertEquals(2, success);
       } else {
         assertEquals(1, success);
@@ -353,7 +348,8 @@ public void testRepeatedIsOptional() throws Exception {
     json.put("required_double", 1.1);
 
     DynamicMessage protoMsg =
-        JsonToProtoMessage.convertJsonToProtoMessage(TestRepeatedIsOptional.getDescriptor(), json, false);
+        JsonToProtoMessage.convertJsonToProtoMessage(
+            TestRepeatedIsOptional.getDescriptor(), json, false);
     AreMatchingFieldsFilledIn(protoMsg, json);
   }
 
@@ -516,7 +512,8 @@ public void testAllowUnknownFields() throws Exception {
   @Test
   public void testAllowUnknownFieldsError() throws Exception {
     JSONObject json = new JSONObject();
-    json.put("double", 1.1);
+    json.put("test_repeated", new JSONArray(new int[] {1, 2, 3, 4, 5}));
+    json.put("string", "hello");
 
     try {
       DynamicMessage protoMsg =
@@ -524,7 +521,7 @@ public void testAllowUnknownFieldsError() throws Exception {
     } catch (IllegalArgumentException e) {
       assertEquals(
           e.getMessage(),
-          "JSONObject has fields unknown to BigQuery: root.double. Set allowUnknownFields to True to allow unknown fields.");
+          "JSONObject has fields unknown to BigQuery: root.string. Set allowUnknownFields to True to allow unknown fields.");
     }
   }
 
@@ -535,14 +532,12 @@ public void testEmptyJSONObject() throws Exception {
       DynamicMessage protoMsg =
           JsonToProtoMessage.convertJsonToProtoMessage(Int64Type.getDescriptor(), json, false);
     } catch (IllegalArgumentException e) {
-      assertEquals(
-          e.getMessage(),
-          "JSONObject is empty.");
+      assertEquals(e.getMessage(), "JSONObject is empty.");
     }
   }
 
   @Test
-  public void testTopLevelMatchSecondLevelNoMatch() throws Exception {
+  public void testAllowUnknownFieldsSecondLevel() throws Exception {
     JSONObject complexLvl2 = new JSONObject();
     complexLvl2.put("no_match", 1);
     JSONObject json = new JSONObject();
@@ -559,6 +554,35 @@ public void testTopLevelMatchSecondLevelNoMatch() throws Exception {
     }
   }
 
+  @Test
+  public void testTopLevelMismatch() throws Exception {
+    JSONObject json = new JSONObject();
+    json.put("no_match", 1.1);
+
+    try {
+      DynamicMessage protoMsg =
+          JsonToProtoMessage.convertJsonToProtoMessage(
+              TopLevelMismatch.getDescriptor(), json, true);
+    } catch (IllegalArgumentException e) {
+      assertEquals(
+          e.getMessage(),
+          "There are no matching fields found for the JSONObject and the protocol buffer descriptor.");
+    }
+  }
+
+  @Test
+  public void testTopLevelMatchSecondLevelMismatch() throws Exception {
+    JSONObject complexLvl2 = new JSONObject();
+    complexLvl2.put("no_match", 1);
+    JSONObject json = new JSONObject();
+    json.put("test_int", 1);
+    json.put("complexLvl2", complexLvl2);
+
+    DynamicMessage protoMsg =
+        JsonToProtoMessage.convertJsonToProtoMessage(ComplexLvl1.getDescriptor(), json, true);
+    AreMatchingFieldsFilledIn(protoMsg, json);
+  }
+
   @Test
   public void testJsonNullValue() throws Exception {
     JSONObject json = new JSONObject();
diff --git a/google-cloud-bigquerystorage/src/test/proto/jsonTest.proto b/google-cloud-bigquerystorage/src/test/proto/jsonTest.proto
index feaf838696..d4c572728d 100644
--- a/google-cloud-bigquerystorage/src/test/proto/jsonTest.proto
+++ b/google-cloud-bigquerystorage/src/test/proto/jsonTest.proto
@@ -112,3 +112,7 @@ message TestRepeatedIsOptional {
   optional double required_double = 1;
   repeated double repeated_double = 2;
 }
+
+message TopLevelMismatch {
+  optional double mismatch_double = 1;
+}