diff --git a/app/models/topic_list.rb b/app/models/topic_list.rb
index 6c2fbc776ca..9578eb11310 100644
--- a/app/models/topic_list.rb
+++ b/app/models/topic_list.rb
@@ -77,7 +77,7 @@ class TopicList
 
   def categories
     @categories ||=
-      topics.map { |t| [t.category&.parent_category, t.category] }.uniq.flatten.compact
+      topics.map { |t| [t.category&.parent_category, t.category] }.flatten.uniq.compact
   end
 
   def load_topics
diff --git a/lib/topic_view.rb b/lib/topic_view.rb
index d52c9f0c37b..1b8ab6eb140 100644
--- a/lib/topic_view.rb
+++ b/lib/topic_view.rb
@@ -727,9 +727,9 @@ class TopicView
   end
 
   def categories
-    categories = [category, category&.parent_category]
-    categories += suggested_topics.categories if suggested_topics
-    categories.compact.uniq
+    @categories ||= [category&.parent_category, category, suggested_topics&.categories].flatten
+      .uniq
+      .compact
   end
 
   protected