How do you test default case for an enum in Java? [duplicate]

This question already has an answer here:

• Mocking Java enum to add a value to test fail case 7 answers

I have code like this:

switch(someEnumValue) {
    case MyEnum.SomeEnum:
        blah();
        break;

    case MyEnum.MoreEnum:
        moreBlah();
        break;

    default:
        throw new IllegalStateException("Unknown enum value.");
    }

How do I test the default case which has been put there for defensive coding purposes?

Do I use a 'null' value for someEnumValue? Or is there some way in Java to sneakily created an instance of an enum which is actually invalid?

I like the defensive coding practice, but when combined with static analysis that checks code coverage and forces things like default cases in switches - it can make testing a little tricky.