diff --git a/src/calibre/ebooks/mobi/debug.py b/src/calibre/ebooks/mobi/debug.py
index 4b3bdbe0c2..32455d0cf8 100644
--- a/src/calibre/ebooks/mobi/debug.py
+++ b/src/calibre/ebooks/mobi/debug.py
@@ -373,7 +373,7 @@ class TagX(object):
         self.eof = ord(raw[3])
 
     def __repr__(self):
-        return 'TAGX(tag=%d, num_values=%d, bitmask=%r, eof=%d)' % (self.tag,
+        return 'TAGX(tag=%02d, num_values=%d, bitmask=%r, eof=%d)' % (self.tag,
                 self.num_values, self.bitmask, self.eof)
 
 class PrimaryIndexRecord(object):