>I don't think Russian is a very logical language, as that word exists, but not the other one. I am wondering if the root is 'kup' ? I am not 100% certain here.

I sometimes think that english language doesn't have rules, it has lists. As for mispairings of this kind, look at this example:

- Man, I've been places!

- Have you been a Helsinki?

Try without "a" as well, sounds equally odd. Also it allows singular only in some cases ("it's a place I've been") but not in others ("man, I've been place!"). There's, I think, only one general instruction: go figure.