diff --git a/beets/autotag/__init__.py b/beets/autotag/__init__.py
index c5fa15b11..3b659b0b6 100644
--- a/beets/autotag/__init__.py
+++ b/beets/autotag/__init__.py
@@ -197,8 +197,8 @@ def apply_metadata(album_info, mapping):
         item.title = track_info.title
 
         if config['per_disc_numbering']:
-            item.track = track_info.medium_index
-            item.tracktotal = track_info.medium_total
+            item.track = track_info.medium_index or track_info.index
+            item.tracktotal = track_info.medium_total or len(album_info.tracks)
         else:
             item.track = track_info.index
             item.tracktotal = len(album_info.tracks)