diff --git a/mercurial/pycompat.py b/mercurial/pycompat.py
--- a/mercurial/pycompat.py
+++ b/mercurial/pycompat.py
@@ -510,7 +510,7 @@
 # This wrapper file are always open in byte mode.
 def unnamedtempfile(mode=None, *args, **kwargs):
     if mode is None:
-        mode = b'w+b'
+        mode = 'w+b'
     else:
         mode = sysstr(mode)
     assert 'b' in mode