How to unit test empty enum?

I have an extension method that calculates due date based on date period type. The method looks like this:

public static DateTime CalculateDueDate(this DateTime date, OffsetType offsetType, int offset)
{
    switch (offsetType)
    {
        case OffsetType.Days:
            return date.AddDays(offset);
        case OffsetType.Weeks:
            return date.AddWeeks(offset);
        case OffsetType.Months:
            return date.AddMonths(offset);
        default:
            throw new ArgumentOutOfRangeException("offsetType", offsetType, null);
    }
}

where OffsetType enum has these possible values:

public enum OffsetType
{
    Months = 1,
    Weeks = 2,
    Days = 3
}

How can I make sure that ArgumentOutOfRangeException is thrown when OffsetType enum is not provided (or provided with invalid value)? Do I even need to worry about unit testing that exception if OffsetType parameter is not null?