Trying to book a flight on united.com/en/ca

At the payment stage, it refuses my credit card:

We are unable to process your request. Please see the message below and make revisions.
- Yukon is not a valid State/Province/Region. Please enter a new State/Province/Region.

I've tried to leave the field empty, and to enter "YT". In those cases, the website just says that there is a problem with my credit or debit card, without dissing on the Yukon. I called my card issuer (Amex), who says there was no attempt to charge my card.

Is this idiocy a known behaviour? Or is it new?