diff -r 3477326461e3 -r cd43fd9f923f hgext/obsolete.py
--- a/hgext/obsolete.py	Wed Aug 15 16:54:48 2012 +0200
+++ b/hgext/obsolete.py	Tue Aug 21 02:37:53 2012 +0200
@@ -707,6 +707,8 @@
         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)