diff --git a/app/mailers/user_notifications.rb b/app/mailers/user_notifications.rb
index cda60d28970..5e8a60af06b 100644
--- a/app/mailers/user_notifications.rb
+++ b/app/mailers/user_notifications.rb
@@ -421,7 +421,7 @@ class UserNotifications < ActionMailer::Base
     end
 
     # category name
-    category = Topic.find_by(id: post.topic_id).category
+    category = Topic.find_by(id: post.topic_id)&.category
     if opts[:show_category_in_subject] && post.topic_id && category && !category.uncategorized?
       show_category_in_subject = category.name