How to unit test the default case of an enum based switch statement

I have a switch statement in a factory that returns a command based on the value of the enum passed in. Something like:

public ICommand Create(EnumType enumType)
{
   switch (enumType)
   {
      case(enumType.Val1):
         return new SomeCommand();
      case(enumType.Val2):
         return new SomeCommand();
      case(enumType.Val3):
         return new SomeCommand();
      default:
         throw new ArgumentOutOfRangeException("Unknown enumType" + enumType);
   }
}

I currently have a switch case for each value in the enum. I have unit tests for each of these cases. How do I unit test that the default case throws an error? Obviously, at the moment I can't pass in an unknown EnumType but who's to say this won't be changed in the future. Is there anyway I can extend or mock the EnumType purely for the sake of the unit test?