diff --git a/src/calibre/ebooks/mobi/writer.py b/src/calibre/ebooks/mobi/writer.py
index 3769320dcd..c9d931c918 100644
--- a/src/calibre/ebooks/mobi/writer.py
+++ b/src/calibre/ebooks/mobi/writer.py
@@ -1634,7 +1634,7 @@ class MobiWriter(object):
         now = int(time.time())
         nrecords = len(self._records)
         self._write(title, pack('>HHIIIIII', 0, 0, now, now, 0, 0, 0, 0),
-            'BOOK', 'MOBI', pack('>IIH', nrecords, 0, nrecords))
+            'BOOK', 'MOBI', pack('>IIH', (2*nrecords)-1, 0, nrecords))
         offset = self._tell() + (8 * nrecords) + 2
         for i, record in enumerate(self._records):
             self._write(pack('>I', offset), '\0', pack('>I', 2*i)[1:])
diff --git a/src/calibre/ebooks/mobi/writer2/main.py b/src/calibre/ebooks/mobi/writer2/main.py
index 2c57a9e461..7421bcb82b 100644
--- a/src/calibre/ebooks/mobi/writer2/main.py
+++ b/src/calibre/ebooks/mobi/writer2/main.py
@@ -550,7 +550,7 @@ class MobiWriter(object):
         now = int(time.time())
         nrecords = len(self.records)
         self.write(title, pack(b'>HHIIIIII', 0, 0, now, now, 0, 0, 0, 0),
-            b'BOOK', b'MOBI', pack(b'>IIH', nrecords, 0, nrecords))
+            b'BOOK', b'MOBI', pack(b'>IIH', (2*nrecords)-1, 0, nrecords))
         offset = self.tell() + (8 * nrecords) + 2
         for i, record in enumerate(self.records):
             self.write(pack(b'>I', offset), b'\0', pack(b'>I', 2*i)[1:])