--- a/mercurial/wireproto.py	Thu Feb 09 15:20:41 2017 -0500
+++ b/mercurial/wireproto.py	Mon Feb 13 20:06:38 2017 +0530
@@ -26,6 +26,7 @@
     exchange,
     peer,
     pushkey as pushkeymod,
+    pycompat,
     streamclone,
     util,
 )
@@ -961,7 +962,7 @@
 
         # write bundle data to temporary file because it can be big
         fd, tempname = tempfile.mkstemp(prefix='hg-unbundle-')
-        fp = os.fdopen(fd, 'wb+')
+        fp = os.fdopen(fd, pycompat.sysstr('wb+'))
         r = 0
         try:
             proto.getfile(fp)