diff --git a/src/calibre/ebooks/mobi/reader/mobi6.py b/src/calibre/ebooks/mobi/reader/mobi6.py
index de1f42f102..75e43a1926 100644
--- a/src/calibre/ebooks/mobi/reader/mobi6.py
+++ b/src/calibre/ebooks/mobi/reader/mobi6.py
@@ -791,6 +791,7 @@ class MobiReader:
             try:
                 num += (ord(data[off:off+1]) & 0x3) + 1
             except TypeError:
+                self.log.warn('Invalid sizeof trailing entries')
                 num += 1
         return num