diff --git a/hgext/win32mbcs.py b/hgext/win32mbcs.py
--- a/hgext/win32mbcs.py
+++ b/hgext/win32mbcs.py
@@ -77,7 +77,7 @@
 
 
 def decode(arg):
-    if isinstance(arg, str):
+    if isinstance(arg, bytes):
         uarg = arg.decode(_encoding)
         if arg == uarg.encode(_encoding):
             return uarg