diff --git a/light9/newtypes.py b/light9/newtypes.py
--- a/light9/newtypes.py
+++ b/light9/newtypes.py
@@ -27,7 +27,7 @@ OutputRange = NewType('OutputRange', Tup
 def _isSubclass2(t1: Type, t2: Type) -> bool:
     """same as issubclass but t1 can be a NewType"""
     if hasattr(t1, '__supertype__'):
-        t1 = t1.__superType__
+        t1 = t1.__supertype__
     return issubclass(t1, t2)