I did some more digging and I realised there is a blacklist of gem names.
It's obvious to me now why the name is not available (it's the name of an existing stdlib module).
It might be worth adding some documentation and linking to it to make it clearer what's going on?
However, I'm sure it's not a common problem.
Thanks anyway and I apologise if I wasted anyone's time.
Kind regards,
Samuel