diff -r 78de43ab585f -r 48dea083f66d mercurial/bundlerepo.py
--- a/mercurial/bundlerepo.py	Thu Feb 09 15:20:41 2017 -0500
+++ b/mercurial/bundlerepo.py	Mon Feb 13 20:06:38 2017 +0530
@@ -272,7 +272,7 @@
                                             suffix=".hg10un")
             self.tempfile = temp
 
-            with os.fdopen(fdtemp, 'wb') as fptemp:
+            with os.fdopen(fdtemp, pycompat.sysstr('wb')) as fptemp:
                 fptemp.write(header)
                 while True:
                     chunk = read(2**18)