diff --git a/src/calibre/ebooks/mobi/writer2/indexer.py b/src/calibre/ebooks/mobi/writer2/indexer.py
index ece96e3a7c..311b4220d9 100644
--- a/src/calibre/ebooks/mobi/writer2/indexer.py
+++ b/src/calibre/ebooks/mobi/writer2/indexer.py
@@ -125,7 +125,7 @@ class IndexEntry(object): # {{{
         buf.append(1)
 
         header = b'TAGX'
-        header += pack(b'>I', len(buf)) # table length
+        header += pack(b'>I', 12+len(buf)) # table length
         header += pack(b'>I', 1) # control byte count
 
         return header + bytes(buf)