diff -r ff461baa9c4e -r 3edd6fedc33a mercurial/extensions.py
--- a/mercurial/extensions.py	Thu Aug 09 01:07:11 2007 +0200
+++ b/mercurial/extensions.py	Thu Aug 09 11:37:47 2007 -0700
@@ -30,6 +30,7 @@
         shortname = name
     if shortname in _extensions:
         return
+    _extensions[shortname] = None
     if path:
         # the module will be loaded in sys.modules
         # choose an unique name so that it doesn't