diff -r b1ebc767563d -r 104914b03b83 mercurial/revset.py
--- a/mercurial/revset.py	Thu Sep 01 22:06:42 2016 +0200
+++ b/mercurial/revset.py	Sun Aug 07 15:01:42 2016 +0900
@@ -2463,7 +2463,7 @@
         else:
             w = 1
         return w + wa, (op, x[1], ta)
-    return 1, x
+    raise ValueError('invalid operator %r' % op)
 
 def optimize(tree):
     _weight, newtree = _optimize(tree, small=True)