--- a/hgext/obsolete.py	Mon Aug 06 19:31:52 2012 +0200
+++ b/hgext/obsolete.py	Tue Aug 21 02:36:33 2012 +0200
@@ -274,6 +274,8 @@
         nc = toproceed.pop()
         for mark in allobjects.get(nc, ()):
             for sub in mark[1]:
+                if sub == nullid:
+                    continue # should not be here!
                 if sub not in seen:
                     seen.add(sub)
                     toproceed.append(sub)