diff -r 9ff5cbfbc26a -r 0e8b76644e20 mercurial/windows.py
--- a/mercurial/windows.py	Tue Feb 27 14:41:24 2018 +0530
+++ b/mercurial/windows.py	Tue Feb 27 14:42:30 2018 +0530
@@ -316,7 +316,7 @@
     # Work around "popen spawned process may not write to stdout
     # under windows"
     # http://bugs.python.org/issue1366
-    command += " 2> %s" % os.devnull
+    command += " 2> %s" % pycompat.bytestr(os.devnull)
     return os.popen(quotecommand(command), mode)
 
 def explainexit(code):