diff --git a/lib/gitlab/markdown/reference_filter.rb b/lib/gitlab/markdown/reference_filter.rb
index d334a891509..c7d4b15d458 100644
--- a/lib/gitlab/markdown/reference_filter.rb
+++ b/lib/gitlab/markdown/reference_filter.rb
@@ -25,7 +25,7 @@ module Gitlab
       end
 
       def self.referenced_by(node)
-        nil
+        raise NotImplementedError, "#{self} does not implement #{__method__}"
       end
 
       # Returns a data attribute String to attach to a reference link