diff -r 89aec1834a86 -r 531332502568 mercurial/keepalive.py
--- a/mercurial/keepalive.py	Sun Apr 24 18:41:23 2016 +0900
+++ b/mercurial/keepalive.py	Fri Sep 29 15:49:43 2017 +0000
@@ -393,7 +393,7 @@
     def read(self, amt=None):
         # the _rbuf test is only in this first if for speed.  It's not
         # logically necessary
-        if self._rbuf and not amt is None:
+        if self._rbuf and amt is not None:
             L = len(self._rbuf)
             if amt > L:
                 amt -= L