diff --git a/hgext/convert/monotone.py b/hgext/convert/monotone.py
--- a/hgext/convert/monotone.py
+++ b/hgext/convert/monotone.py
@@ -93,9 +93,9 @@
         kwargs = pycompat.byteskwargs(kwargs)
         command = []
         for k, v in kwargs.iteritems():
-            command.append("%s:%s" % (len(k), k))
+            command.append("%d:%s" % (len(k), k))
             if v:
-                command.append("%s:%s" % (len(v), v))
+                command.append("%d:%s" % (len(v), v))
         if command:
             command.insert(0, 'o')
             command.append('e')