Index: iweb.py
===================================================================
--- iweb.py     (revision 18545)
+++ iweb.py     (working copy)
@@ -41,10 +41,14 @@
         """
 
 # Is there a better way to do this than this funky extra class?
+NotSpecified = object()
 class SpecialAdaptInterfaceClass(interface.InterfaceClass):
     # A special adapter for IResource to handle the extra step of adapting
     # from IOldNevowResource-providing resources.
-    def __call__(self, other, alternate=None):
+    def __call__(self, other, alternate=NotSpecified):
+        if alternate == NotSpecified:
+            return super(SpecialAdaptInterfaceClass, self).__call__(other)
+
         result = super(SpecialAdaptInterfaceClass, self).__call__(other, alternate)
         if result is not alternate:
             return result