diff --git a/data/model/tag.py b/data/model/tag.py
index 0570f831a..f533e9a08 100644
--- a/data/model/tag.py
+++ b/data/model/tag.py
@@ -143,10 +143,10 @@ def garbage_collect_tags(repo):
 
   if len(tags_to_delete) > 0:
     with db_transaction():
-      manifests_to_delete = (TagManifest
-                             .select(TagManifest.id)
-                             .join(RepositoryTag)
-                             .where(RepositoryTag.id << tags_to_delete))
+      manifests_to_delete = list(TagManifest
+                                 .select(TagManifest.id)
+                                 .join(RepositoryTag)
+                                 .where(RepositoryTag.id << tags_to_delete))
 
       num_deleted_manifests = (TagManifest
                                .delete()