--- a/vregistry.py	Wed Feb 18 15:48:57 2009 +0100
+++ b/vregistry.py	Wed Feb 18 15:49:59 2009 +0100
@@ -237,6 +237,7 @@
         # as such because we need to compose registered in some cases
         vobject = obj.registered.im_func(obj, self)
         assert not vobject in vobjects
+        assert callable(vobject.__select__), vobject
         vobjects.append(vobject)
         try:
             vname = vobject.__name__