diff --git a/src/calibre/ebooks/mobi/reader/mobi6.py b/src/calibre/ebooks/mobi/reader/mobi6.py
index f3bb969272..de1f42f102 100644
--- a/src/calibre/ebooks/mobi/reader/mobi6.py
+++ b/src/calibre/ebooks/mobi/reader/mobi6.py
@@ -788,7 +788,10 @@ class MobiReader:
             flags >>= 1
         if self.book_header.extra_flags & 1:
             off = size - num - 1
-            num += (ord(data[off:off+1]) & 0x3) + 1
+            try:
+                num += (ord(data[off:off+1]) & 0x3) + 1
+            except TypeError:
+                num += 1
         return num
 
     def warn_about_trailing_entry_corruption(self):