How to imitate nonexistent enum values in tests?

I have following method:

public void methodToTest(Locale locale){
            switch (locale) {
                case EN:
                    ...
                    break;
                case FR:
                    ...
                    break;
                default:
                    throw new IllegalArgumentException(String.format("Unexpected locale: %s", String.valueOf(locale)));
            }
}

and following enum:

public enum Locale {EN,FR}

Please help me to cover the default branch of switch. I have not ideas how to realize it.