diff --git a/app/helpers/message_format_helper.rb b/app/helpers/message_format_helper.rb
index 3dd8d8f23..1e89c56c1 100644
--- a/app/helpers/message_format_helper.rb
+++ b/app/helpers/message_format_helper.rb
@@ -2,7 +2,8 @@ module MessageFormatHelper
   include RegexHelper
 
   def transform_user_mention_content(message_content)
-    message_content.gsub(MENTION_REGEX, '\1')
+    # attachment message without content, message_content is nil
+    message_content.presence ? message_content.gsub(MENTION_REGEX, '\1') : ''
   end
 
   def render_message_content(message_content)