diff --git a/app/assets/javascripts/discourse/models/topic-list.js.es6 b/app/assets/javascripts/discourse/models/topic-list.js.es6
index 5346fd977ea..b9076f078d0 100644
--- a/app/assets/javascripts/discourse/models/topic-list.js.es6
+++ b/app/assets/javascripts/discourse/models/topic-list.js.es6
@@ -111,7 +111,7 @@ TopicList.reopenClass({
         if (p.primary_group_id) {
           p.primary_group = groups[p.primary_group_id];
           if (p.primary_group) {
-            p.extraClasses += ` group-${p.primary_group.name}`;
+            p.extraClasses = `${p.extraClasses||''} group-${p.primary_group.name}`;
           }
         }
       });