Yoast

@matt can you please review this?
We don't have to mandate a function here. A user could choose not to pass in a callback and that would be fine. It wouldn't be a smart thing to do generally, but I don't think it merits throwing an error as the call method will just set it to null. Really not a big thing and we'll never use this endpoint without a callback, but it is extra code we don't really need IMO.