diff --git a/src/templates/object.derived b/src/templates/object.derived
--- a/src/templates/object.derived
+++ b/src/templates/object.derived
@@ -149,9 +149,7 @@
     PyObject impl = self_type.lookup("__len__");
     if (impl != null) {
       PyObject res = impl.__get__(this,self_type).__call__();
-      if (res instanceof PyInteger)
-         return ((PyInteger)res).getValue();
-      throw Py.TypeError("__len__ should return a int");
+      return res.asInt();
     }
     return super.__len__();
   }