PKPaymentRequest conditionally invalid

Question

When creating a paymentRequest, configuring as such works:

let paymentRequest = PKPaymentRequest()
...
paymentRequest.countryCode = "US";
paymentRequest.currencyCode = "USD";
...

However, configuring by using "UK" and "GBP" doesnt work. Im probably missing something, is there a ruleset somewhere? Im simply trying to make a payment in pounds.

More specifically (code from Braintree ) :

// Example: Promote PKPaymentAuthorizationViewController to optional so that we can verify
// that our paymentRequest is valid. Otherwise, an invalid paymentRequest would crash our app.

 if let vca = PKPaymentAuthorizationViewController(paymentRequest: paymentRequest) as PKPaymentAuthorizationViewController?
 {
    ...logic...
 } else {
    ->this is what i get when using pounds or anything else<-
 }