--- a/mercurial/localrepo.py	Tue Dec 05 16:33:40 2006 +0100
+++ b/mercurial/localrepo.py	Tue Dec 05 13:41:42 2006 -0800
@@ -376,6 +376,8 @@
             key = self.dirstate.parents()[0]
             if key == nullid:
                 raise repo.RepoError(_("no revision checked out"))
+        elif key == 'null':
+            return nullid
         n = self.changelog._match(key)
         if n:
             return n