I'm sorry, I haven't looked at this in a while now and all I can tell you is that we included `user_id` in the custom_data block to get things working for us. If we didn't include it there, where would we setup `user_id`? I guess `user.id` doesn't return a string if you are using MongoDB so we'd need to setup `user_id` somewhere in the config? (So no, I don't think the comment on line 40 is enough - I think we need to tell them where in the config they are expected to set `user_id`) 

It's also possible that since I haven't thought about this in a while, I am missing the point somehow. Apologies if this is the case.