diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -349,7 +349,7 @@ class ui(object):
                 user = '%s@%s' % (util.getuser(), socket.getfqdn())
             except KeyError:
                 raise util.Abort(_("Please specify a username."))
-            self.warn(_("No username found, using '%s' instead\n" % user))
+            self.warn(_("No username found, using '%s' instead\n") % user)
         return user
 
     def shortuser(self, user):