diff --git a/src/calibre/ebooks/mobi/writer2/main.py b/src/calibre/ebooks/mobi/writer2/main.py
index 4737224336..27c4838a4b 100644
--- a/src/calibre/ebooks/mobi/writer2/main.py
+++ b/src/calibre/ebooks/mobi/writer2/main.py
@@ -387,7 +387,8 @@ class MobiWriter(object):
         self.records.append(self.kf8.record0)
         self.records.extend(self.kf8.records[1:])
 
-        first_image_record if first_image_record else len(self.records)
+        first_image_record = (first_image_record if first_image_record else
+                len(self.records))
 
         header_fields = {k:getattr(self.kf8, k) for k in HEADER_FIELDS}
 
@@ -407,6 +408,8 @@ class MobiWriter(object):
                 'ncx_index':'primary_index_record_idx',
                 }.iteritems():
             header_fields[k] = getattr(self, v)
+        if header_fields['ncx_index'] is None:
+            header_fields['ncx_index'] = NULL_INDEX
 
         for x in ('skel', 'chunk', 'guide'):
             header_fields[x+'_index'] = NULL_INDEX
diff --git a/src/calibre/ebooks/mobi/writer8/mobi.py b/src/calibre/ebooks/mobi/writer8/mobi.py
index 7cdfd5a4b1..18f19a4084 100644
--- a/src/calibre/ebooks/mobi/writer8/mobi.py
+++ b/src/calibre/ebooks/mobi/writer8/mobi.py
@@ -229,9 +229,9 @@ class KF8Book(object):
             setattr(self, x, getattr(resources, x))
 
         self.first_resource_record = NULL_INDEX
+        before = len(self.records)
         if resources.records:
             self.first_resource_record = len(self.records)
-            before = len(self.records)
             if not for_joint:
                 resources.serialize(self.records, writer.used_images)
         self.num_of_resources = len(self.records) - before