diff --git a/hgext/convert/monotone.py b/hgext/convert/monotone.py
--- a/hgext/convert/monotone.py
+++ b/hgext/convert/monotone.py
@@ -102,7 +102,7 @@
 
         command.append('l')
         for arg in args:
-            command += "%d:%s" % (len(arg), arg)
+            command += pycompat.bytestr("%d:%s" % (len(arg), arg))
         command.append('e')
         command = ''.join(command)