diff --git a/app/assets/javascripts/discourse/models/topic.js.es6 b/app/assets/javascripts/discourse/models/topic.js.es6
index 00ec3934e65..e769d7e87f1 100644
--- a/app/assets/javascripts/discourse/models/topic.js.es6
+++ b/app/assets/javascripts/discourse/models/topic.js.es6
@@ -227,7 +227,7 @@ const Topic = RestModel.extend({
 
   @computed("tags")
   tagClasses(tags) {
-    return tags.map(t => `tag-${t}`).join(" ");
+    return tags && tags.map(t => `tag-${t}`).join(" ");
   },
 
   @computed("url")