diff --git a/migration/tables/content_items.py b/migration/tables/content_items.py
index 3adfebcd..16147fd6 100644
--- a/migration/tables/content_items.py
+++ b/migration/tables/content_items.py
@@ -94,7 +94,7 @@ def migrate(entry, users_by_oid, topics_by_oid):
 	mainTopic = topics_by_oid.get(category)
 	if mainTopic:
 		r['mainTopic'] = mainTopic["slug"]
-	topic_oids = set(category)
+	topic_oids = set([category])
 	topic_oids.update(entry.get("tags", []))
 	for oid in topic_oids:
 		if oid in topics_by_oid: