diff -r 8474005fcfe2 -r 0a1fb171dc1d mercurial/localrepo.py
--- a/mercurial/localrepo.py	Sun Apr 01 23:29:51 2018 -0700
+++ b/mercurial/localrepo.py	Tue Apr 03 15:08:09 2018 -0700
@@ -1021,7 +1021,7 @@
                 pass
 
     def lookup(self, key):
-        return self[key].node()
+        return scmutil.revsymbol(self, key).node()
 
     def lookupbranch(self, key, remote=None):
         repo = remote or self