diff -r 012780620d4f -r ef7068173a22 mercurial/commands.py
--- a/mercurial/commands.py	Fri Apr 12 16:55:34 2013 -0400
+++ b/mercurial/commands.py	Thu Apr 11 14:44:22 2013 +0200
@@ -5601,7 +5601,7 @@
         # don't allow tagging the null rev
         if (not opts.get('remove') and
             scmutil.revsingle(repo, rev_).rev() == nullrev):
-            raise util.Abort(_("null revision specified"))
+            raise util.Abort(_("cannot tag null revision"))
 
         repo.tag(names, r, message, opts.get('local'), opts.get('user'), date)
     finally: