--- a/mercurial/util.py	Mon Feb 13 20:06:38 2017 +0530
+++ b/mercurial/util.py	Mon Feb 13 22:15:28 2017 +0530
@@ -809,7 +809,7 @@
     inname, outname = None, None
     try:
         infd, inname = tempfile.mkstemp(prefix='hg-filter-in-')
-        fp = os.fdopen(infd, 'wb')
+        fp = os.fdopen(infd, pycompat.sysstr('wb'))
         fp.write(s)
         fp.close()
         outfd, outname = tempfile.mkstemp(prefix='hg-filter-out-')