diff -r fdd22bf6398f -r 92171562c7f3 mercurial/localrepo.py
--- a/mercurial/localrepo.py	Wed Apr 04 14:31:09 2018 -0700
+++ b/mercurial/localrepo.py	Wed Apr 04 14:57:58 2018 -0700
@@ -1023,7 +1023,7 @@
         if key in self.branchmap():
             return key
 
-        return self[key].branch()
+        return scmutil.revsymbol(self, key).branch()
 
     def known(self, nodes):
         cl = self.changelog