diff --git a/pythonnet/src/runtime/assemblymanager.cs b/pythonnet/src/runtime/assemblymanager.cs
index 80c838df2..3fc4a5feb 100644
--- a/pythonnet/src/runtime/assemblymanager.cs
+++ b/pythonnet/src/runtime/assemblymanager.cs
@@ -283,7 +283,7 @@ static void ScanAssembly(Assembly assembly) {
                     namespaces[ns].Add(assembly, String.Empty);
                 }
 
-                if (t.IsGenericTypeDefinition) {
+                if (ns != null && t.IsGenericTypeDefinition) {
                     GenericUtil.Register(t);
                 }
             }