It is an error as error 418 is returned if you attempt to communicate with a teapot over the "Hyper Text Coffee Pot Control Protocol", which, as the name implies, is only to be used to talk to coffee pots[0]. The protocol was extended in 2014 for teapot compatibility[1].

0: https://tools.ietf.org/html/rfc2324

1: https://tools.ietf.org/html/rfc7168