mercurial/commands.py

--- a/mercurial/commands.py	Mon Jan 07 15:25:41 2019 -0800
+++ b/mercurial/commands.py	Mon Jan 07 19:03:23 2019 -0500
@@ -6098,7 +6098,7 @@
         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', '')}