diff --git a/src/runtime/metatype.cs b/src/runtime/metatype.cs
index 014c5917c..6c268dbcb 100644
--- a/src/runtime/metatype.cs
+++ b/src/runtime/metatype.cs
@@ -145,7 +145,9 @@ public static IntPtr tp_new(IntPtr tp, IntPtr args, IntPtr kw)
                 return IntPtr.Zero;
             }
 
-            var flags = TypeFlags.Default;
+            var flags = (TypeFlags)Util.ReadCLong(type, TypeOffset.tp_flags);
+            if (!flags.HasFlag(TypeFlags.Ready))
+                throw new NotSupportedException("PyType.tp_new returned an incomplete type");
             flags |= TypeFlags.HasClrInstance;
             flags |= TypeFlags.HeapType;
             flags |= TypeFlags.BaseType;
@@ -170,8 +172,7 @@ public static IntPtr tp_new(IntPtr tp, IntPtr args, IntPtr kw)
             IntPtr gc = Marshal.ReadIntPtr(base_type, Offsets.tp_clr_inst);
             Marshal.WriteIntPtr(type, Offsets.tp_clr_inst, gc);
 
-            if (Runtime.PyType_Ready(type) != 0)
-                throw PythonException.ThrowLastAsClrException();
+            Runtime.PyType_Modified(new BorrowedReference(type));
 
             return type;
         }