--- a/mercurial/commands.py	Sun Dec 30 16:11:06 2018 +0100
+++ b/mercurial/commands.py	Wed Jan 02 09:41:04 2019 +0900
@@ -6038,7 +6038,7 @@
         brev = rev
         if rev:
             repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
-        ctx = scmutil.revsingle(repo, rev, rev)
+        ctx = scmutil.revsingle(repo, rev, default=None)
         rev = ctx.rev()
         hidden = ctx.hidden()
         overrides = {('ui', 'forcemerge'): opts.get(r'tool', '')}