diff --git a/hgext/convert/common.py b/hgext/convert/common.py
--- a/hgext/convert/common.py
+++ b/hgext/convert/common.py
@@ -339,7 +339,7 @@ class commandline(object):
                 pass
         cmdline = [util.shellquote(arg) for arg in cmdline]
         if not self.ui.debugflag:
-            cmdline += ['2>', os.devnull]
+            cmdline += ['2>', pycompat.bytestr(os.devnull)]
         cmdline = ' '.join(cmdline)
         return cmdline
 
diff --git a/mercurial/windows.py b/mercurial/windows.py
--- a/mercurial/windows.py
+++ b/mercurial/windows.py
@@ -316,7 +316,7 @@ def popen(command, mode='r'):
     # Work around "popen spawned process may not write to stdout
     # under windows"
     # http://bugs.python.org/issue1366
-    command += " 2> %s" % os.devnull
+    command += " 2> %s" % pycompat.bytestr(os.devnull)
     return os.popen(quotecommand(command), mode)
 
 def explainexit(code):