My reasoning was that an app writer who made an error would have a really quiet fail: the cacheing wouldn't function as expected, but discovering that would be a long way from the misconfigure. In the case of no-store that could be really bad (from, admittedly, a certain perspective).

I'd like to at least put a warning and ignore that key. It does complicate the Response method a little bit: if the CC header is empty, it shouldn't be added.