Not accepting UK as a country

[MarkRosemaker]

"UK" does not seem to be accepted as a country, but "GB" is.

When I run:

var num libphonenumber.PhoneNumber
err := libphonenumber.ParseToNumber("0123456789", "GB", &num)
if err != nil {
	fmt.Println("error with GB:", err)
	return
}
err = libphonenumber.ParseToNumber("0123456789", "UK", &num)
if err != nil {
	fmt.Println("error with UK:", err)
	return
}

I get:
error with UK: invalid country code

However, according to some resources I've checked, +44 is always referred to the calling code of the United Kingdom.