diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs
index ed3da3ac632967..48f952a8a19a78 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs
@@ -15,6 +15,14 @@ namespace System.Text.Json
     /// or a type's converter, if the current instance is a <see cref="JsonClassInfo.PropertyInfoForClassInfo"/>.
     internal sealed class JsonPropertyInfo<T> : JsonPropertyInfo
     {
+        /// <summary>
+        /// Returns true if the property's converter is external (a user's custom converter)
+        /// and the type to convert is not the same as the declared property type (polymorphic).
+        /// Used to determine whether to perform additional validation on the value returned by the
+        /// converter on deserialization.
+        /// </summary>
+        private bool _converterIsExternalAndPolymorphic;
+
         public Func<object, T>? Get { get; private set; }
         public Action<object, T>? Set { get; private set; }
 
@@ -91,6 +99,7 @@ public override void Initialize(
                     }
             }
 
+            _converterIsExternalAndPolymorphic = !converter.IsInternalConverter && DeclaredPropertyType != converter.TypeToConvert;
             GetPolicies(ignoreCondition, parentTypeNumberHandling, defaultValueIsNull: Converter.CanBeNull);
         }
 
@@ -230,7 +239,9 @@ public override bool ReadJsonAndSetMember(object obj, ref ReadStack state, ref U
                     success = Converter.TryRead(ref reader, RuntimePropertyType!, Options, ref state, out T value);
                     if (success)
                     {
-                        if (!Converter.IsInternalConverter)
+#if !DEBUG
+                        if (_converterIsExternalAndPolymorphic)
+#endif
                         {
                             if (value != null)
                             {
@@ -245,6 +256,7 @@ public override bool ReadJsonAndSetMember(object obj, ref ReadStack state, ref U
                                 ThrowHelper.ThrowInvalidOperationException_DeserializeUnableToAssignNull(DeclaredPropertyType);
                             }
                         }
+
                         Set!(obj, value!);
                     }
                 }
diff --git a/src/libraries/System.Text.Json/tests/AssertHelper.cs b/src/libraries/System.Text.Json/tests/AssertHelper.cs
deleted file mode 100644
index 0821027726a8aa..00000000000000
--- a/src/libraries/System.Text.Json/tests/AssertHelper.cs
+++ /dev/null
@@ -1,20 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.Collections.Generic;
-using Xunit;
-
-namespace System.Text.Json.Tests
-{
-    public static class AssertHelper
-    {
-        public static void ValidateJson(IEnumerable<string> expectedProperties, string json)
-        {
-            Assert.StartsWith("{", json);
-            Assert.EndsWith("}", json);
-            foreach (string expectedProperty in expectedProperties)
-                Assert.Contains(expectedProperty, json);
-        }
-
-    }
-}
diff --git a/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.Object.cs b/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.Object.cs
index 1ecdfebc935ada..0ab7ad020de5d2 100644
--- a/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.Object.cs
+++ b/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.Object.cs
@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Text.Json.Tests;
 using Xunit;
 
 namespace System.Text.Json.Serialization.Tests
@@ -396,15 +395,14 @@ private class ClassWithPrimitives
         [Fact]
         public static void ClassWithPrimitivesObjectConverter()
         {
-            string[] expected = new[]
-            {
-                @"""MyIntProperty"":123",
-                @"""MyBoolProperty"":true",
-                @"""MyStringProperty"":""Hello""",
-                @"""MyIntField"":321",
-                @"""MyBoolField"":true",
-                @"""MyStringField"":""World"""
-            };
+            string expected = @"{
+""MyIntProperty"":123,
+""MyBoolProperty"":true,
+""MyStringProperty"":""Hello"",
+""MyIntField"":321,
+""MyBoolField"":true,
+""MyStringField"":""World""
+}";
 
             string json;
             var converter = new PrimitiveConverter();
@@ -428,7 +426,7 @@ public static void ClassWithPrimitivesObjectConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(6, converter.WriteCallCount);
-                AssertHelper.ValidateJson(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
             {
                 var obj = JsonSerializer.Deserialize<ClassWithPrimitives>(json, options);
@@ -459,15 +457,14 @@ private class ClassWithNullablePrimitives
         [Fact]
         public static void ClassWithNullablePrimitivesObjectConverter()
         {
-            string[] expected = new[]
-            {
-                @"""MyIntProperty"":123",
-                @"""MyBoolProperty"":true",
-                @"""MyStringProperty"":""Hello""",
-                @"""MyIntField"":321",
-                @"""MyBoolField"":true",
-                @"""MyStringField"":""World"""
-            };
+            string expected = @"{
+""MyIntProperty"":123,
+""MyBoolProperty"":true,
+""MyStringProperty"":""Hello"",
+""MyIntField"":321,
+""MyBoolField"":true,
+""MyStringField"":""World""
+}";
 
             string json;
             var converter = new PrimitiveConverter();
@@ -491,7 +488,7 @@ public static void ClassWithNullablePrimitivesObjectConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(6, converter.WriteCallCount);
-                AssertHelper.ValidateJson(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
             {
                 var obj = JsonSerializer.Deserialize<ClassWithNullablePrimitives>(json, options);
diff --git a/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.ValueTypedMember.cs b/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.ValueTypedMember.cs
index a4fa41de979f79..9501b00a04a870 100644
--- a/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.ValueTypedMember.cs
+++ b/src/libraries/System.Text.Json/tests/Serialization/CustomConverterTests/CustomConverterTests.ValueTypedMember.cs
@@ -208,7 +208,7 @@ public static void ValueTypedMemberToInterfaceConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -240,7 +240,7 @@ public static void ValueTypedMemberToObjectConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -272,7 +272,7 @@ public static void NullableValueTypedMemberToInterfaceConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -304,7 +304,7 @@ public static void NullableValueTypedMemberToObjectConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -334,7 +334,7 @@ public static void NullableValueTypedMemberWithNullsToInterfaceConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -367,7 +367,7 @@ public static void NullableValueTypedMemberWithNullsToObjectConverter()
                 json = JsonSerializer.Serialize(obj, options);
 
                 Assert.Equal(4, converter.WriteCallCount);
-                Assert.Equal(expected, json);
+                JsonTestHelper.AssertJsonEqual(expected, json);
             }
 
             {
@@ -380,6 +380,5 @@ public static void NullableValueTypedMemberWithNullsToObjectConverter()
                 Assert.Null(obj.MyRefTypedField);
             }
         }
-
     }
 }
diff --git a/src/libraries/System.Text.Json/tests/Serialization/TestClasses/TestClasses.ValueTypedMember.cs b/src/libraries/System.Text.Json/tests/Serialization/TestClasses/TestClasses.ValueTypedMember.cs
index ab7384afce90e1..31e51649cf3a03 100644
--- a/src/libraries/System.Text.Json/tests/Serialization/TestClasses/TestClasses.ValueTypedMember.cs
+++ b/src/libraries/System.Text.Json/tests/Serialization/TestClasses/TestClasses.ValueTypedMember.cs
@@ -1,8 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Collections.Generic;
-using System.Linq;
 using Xunit;
 
 namespace System.Text.Json.Serialization.Tests
@@ -105,5 +103,4 @@ public OtherRTMember(string value)
             Value = value;
         }
     }
-
 }
diff --git a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj
index adb637c9244b8f..2f8a4ffcbae617 100644
--- a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj
+++ b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj
@@ -9,7 +9,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonTestPath)System\IO\WrappedMemoryStream.cs" Link="CommonTest\System\IO\WrappedMemoryStream.cs" />
-    <Compile Include="AssertHelper.cs" />
     <Compile Include="BitStackTests.cs" />
     <Compile Include="BufferFactory.cs" />
     <Compile Include="BufferSegment.cs" />