diff --git a/mercurial/subrepo.py b/mercurial/subrepo.py
--- a/mercurial/subrepo.py
+++ b/mercurial/subrepo.py
@@ -1434,7 +1434,7 @@
         # which is mostly progress and useful info
         errpipe = None
         if self.ui.quiet:
-            errpipe = open(os.devnull, b'w')
+            errpipe = open(os.devnull, 'w')
         if self.ui._colormode and len(commands) and commands[0] == b"diff":
             # insert the argument in the front,
             # the end of git diff arguments is used for paths