--- a/mercurial/revset.py	Mon Apr 04 16:21:57 2011 -0500
+++ b/mercurial/revset.py	Mon Apr 04 16:21:59 2011 -0500
@@ -207,7 +207,7 @@
     First parent of changesets in set, or the working directory.
     """
     if x is None:
-        p = repo[x].parents()[0].rev()
+        p = repo[x].p1().rev()
         return [r for r in subset if r == p]
 
     ps = set()