--- a/mercurial/commands.py	Thu Feb 09 17:04:17 2012 -0600
+++ b/mercurial/commands.py	Thu Feb 09 17:54:47 2012 -0600
@@ -5723,6 +5723,7 @@
             raise util.Abort(_("uncommitted local changes"))
         if not rev:
             rev = repo[repo[None].branch()].rev()
+        mergemod._checkunknown(repo, repo[None], repo[rev])
 
     if clean:
         ret = hg.clean(repo, rev)