diff --git a/src/Tests/FunctionalTests/DynamicInterfaceCasting/Program.cs b/src/Tests/FunctionalTests/DynamicInterfaceCasting/Program.cs
index 5f23cec16..f026e2d67 100644
--- a/src/Tests/FunctionalTests/DynamicInterfaceCasting/Program.cs
+++ b/src/Tests/FunctionalTests/DynamicInterfaceCasting/Program.cs
@@ -38,6 +38,17 @@
 IList<List<Point>> list2 = new List<List<Point>>();
 instance2.IterableOfPointIterablesProperty = list2;
 
+// Ensure that these don't crash but return null
+if ((IWinRTObject)instance as IList<Point> != null)
+{
+    return 105;
+}
+
+if ((IWinRTObject)instance as IList<ManagedClass> != null)
+{
+    return 106;
+}
+
 return 100;
 
 object SetAndGetBoxedValue(TestComponentCSharp.Class instance, object val)
@@ -45,3 +56,8 @@ object SetAndGetBoxedValue(TestComponentCSharp.Class instance, object val)
     instance.ObjectProperty = val;
     return instance.ObjectProperty;
 }
+
+class ManagedClass
+{
+    public int Number { get; set; }
+}
\ No newline at end of file
diff --git a/src/WinRT.Runtime/IWinRTObject.net5.cs b/src/WinRT.Runtime/IWinRTObject.net5.cs
index faf4e4481..c75f85972 100644
--- a/src/WinRT.Runtime/IWinRTObject.net5.cs
+++ b/src/WinRT.Runtime/IWinRTObject.net5.cs
@@ -159,6 +159,13 @@ internal sealed bool IsInterfaceImplementedFallback(RuntimeTypeHandle interfaceT
                 }
             }
 
+            // Make sure we are going to do a QI on a WinRT interface,
+            // otherwise we can get a helper type for a non WinRT type.
+            if (!Projections.IsTypeWindowsRuntimeType(type))
+            {
+                return false;
+            }
+
             Type helperType = type.FindHelperType();
             if (helperType is null || !helperType.IsInterface)
             {