diff --git a/src/calibre/ebooks/metadata/mobi.py b/src/calibre/ebooks/metadata/mobi.py
index 2da9f74961..c9c7d80902 100644
--- a/src/calibre/ebooks/metadata/mobi.py
+++ b/src/calibre/ebooks/metadata/mobi.py
@@ -360,7 +360,7 @@ class MetadataUpdater(object):
 
             if kindle_pdoc and kindle_pdoc in mi.tags:
                 added_501 = True
-                update_exth_record((501, str('PDOC')))
+                update_exth_record((501, b'PDOC'))
 
         if mi.pubdate:
             update_exth_record((106, str(mi.pubdate).encode(self.codec, 'replace')))
diff --git a/src/calibre/ebooks/mobi/writer2/main.py b/src/calibre/ebooks/mobi/writer2/main.py
index 1705a5a342..4339532ee3 100644
--- a/src/calibre/ebooks/mobi/writer2/main.py
+++ b/src/calibre/ebooks/mobi/writer2/main.py
@@ -535,7 +535,7 @@ class MobiWriter(object):
             nrecs += 1
 
         # Write cdetype
-        if not self.is_periodical:
+        if not self.is_periodical and not self.opts.share_not_sync:
             exth.write(pack(b'>II', 501, 12))
             exth.write(b'EBOK')
             nrecs += 1