diff -r 74e691b141c4 -r 0aca2695a110 mercurial/commands.py
--- a/mercurial/commands.py	Wed Dec 07 12:56:44 2011 +0100
+++ b/mercurial/commands.py	Wed Dec 07 11:22:57 2011 -0600
@@ -4043,7 +4043,7 @@
                                  % branch,
                                  hint=_("run 'hg heads' to see all heads"))
             msg = _('there is nothing to merge')
-            if parent != repo.lookup(repo[None].branch()):
+            if parent != repo.lookup(branch):
                 msg = _('%s - use "hg update" instead') % msg
             raise util.Abort(msg)