diff -r cf137319c6cd -r c8ee34917045 mercurial/commands.py
--- a/mercurial/commands.py	Sat Apr 21 08:24:10 2012 -0700
+++ b/mercurial/commands.py	Mon Apr 23 12:09:54 2012 +0200
@@ -5664,7 +5664,7 @@
         c = repo[None]
         if c.modified() or c.added() or c.removed():
             raise util.Abort(_("uncommitted local changes"))
-        if not rev:
+        if rev is None:
             rev = repo[repo[None].branch()].rev()
         mergemod._checkunknown(repo, repo[None], repo[rev])