diff --git a/hgext/convert/monotone.py b/hgext/convert/monotone.py
--- a/hgext/convert/monotone.py
+++ b/hgext/convert/monotone.py
@@ -154,7 +154,7 @@
         retval = []
         while True:
             commandnbr, stream, length, output = self.mtnstdioreadpacket()
-            self.ui.debug('mtn: read packet %s:%s:%s\n' %
+            self.ui.debug('mtn: read packet %s:%s:%d\n' %
                 (commandnbr, stream, length))
 
             if stream == 'l':