--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -437,9 +437,12 @@ class localrepository(repo.repository):
 
     def lookup(self, key):
         if key == '.':
-            key = self.dirstate.parents()[0]
+            key, second = self.dirstate.parents()
             if key == nullid:
                 raise repo.RepoError(_("no revision checked out"))
+            if second != nullid:
+                self.ui.warn(_("warning: working directory has two parents, "
+                               "tag '.' uses the first\n"))
         elif key == 'null':
             return nullid
         n = self.changelog._match(key)