diff --git a/src/calibre/ebooks/mobi/writer2/main.py b/src/calibre/ebooks/mobi/writer2/main.py
index 4339532ee3..655aa12c9e 100644
--- a/src/calibre/ebooks/mobi/writer2/main.py
+++ b/src/calibre/ebooks/mobi/writer2/main.py
@@ -601,7 +601,7 @@ class MobiWriter(object):
         Write the PalmDB header
         '''
         title = ascii_filename(unicode(self.oeb.metadata.title[0])).replace(
-                ' ', '_')[:32]
+                ' ', '_')[:31]
         title = title + (b'\0' * (32 - len(title)))
         now = int(time.time())
         nrecords = len(self.records)