diff --git a/src/calibre/customize/builtins.py b/src/calibre/customize/builtins.py
index e8d5c5fc91..e64bba428d 100644
--- a/src/calibre/customize/builtins.py
+++ b/src/calibre/customize/builtins.py
@@ -445,7 +445,7 @@ class LRFMetadataWriter(MetadataWriterPlugin):
 class MOBIMetadataWriter(MetadataWriterPlugin):
 
     name        = 'Set MOBI metadata'
-    file_types  = set(['mobi', 'prc', 'azw', 'azw4'])
+    file_types  = set(['mobi', 'prc', 'azw', 'azw3', 'azw4'])
     description = _('Set metadata in %s files')%'MOBI'
     author      = 'Marshall T. Vandegrift'