diff -r 48dea083f66d -r 82f1ef8b4477 mercurial/changegroup.py
--- a/mercurial/changegroup.py	Mon Feb 13 20:06:38 2017 +0530
+++ b/mercurial/changegroup.py	Mon Feb 13 22:15:28 2017 +0530
@@ -26,6 +26,7 @@
     error,
     mdiff,
     phases,
+    pycompat,
     util,
 )
 
@@ -98,7 +99,7 @@
                 fh = open(filename, "wb", 131072)
         else:
             fd, filename = tempfile.mkstemp(prefix="hg-bundle-", suffix=".hg")
-            fh = os.fdopen(fd, "wb")
+            fh = os.fdopen(fd, pycompat.sysstr("wb"))
         cleanup = filename
         for c in chunks:
             fh.write(c)