diff -r f5aa20e177c0 -r 52e4ac3e63f7 mercurial/commands.py
--- a/mercurial/commands.py	Sun Jul 25 20:01:33 2010 -0500
+++ b/mercurial/commands.py	Tue Jul 27 14:40:25 2010 -0400
@@ -3660,6 +3660,8 @@
     for n in names:
         if n in ['tip', '.', 'null']:
             raise util.Abort(_('the name \'%s\' is reserved') % n)
+        if not n:
+            raise util.Abort(_('tag names cannot consist entirely of whitespace'))
     if opts.get('rev') and opts.get('remove'):
         raise util.Abort(_("--rev and --remove are incompatible"))
     if opts.get('rev'):