public void testAdd_Field() {
        MutablePeriod test = new MutablePeriod(1, 2, 3, 4, 5, 6, 7, 8);
        test.add(DurationFieldType.years(), 10);
        assertEquals(11, test.getYears());
        
        test = new MutablePeriod(0, 0, 0, 0, 0, 0, 0, 1, PeriodType.millis());
        test.add(DurationFieldType.years(), 0);
        assertEquals(0, test.getYears());
        assertEquals(1, test.getMillis());
        
        test = new MutablePeriod(1, 2, 3, 4, 5, 6, 7, 8);
        try {
            test.add(null, 0);
            fail();
        } catch (IllegalArgumentException ex) {}
        
        test = new MutablePeriod(1, 2, 3, 4, 5, 6, 7, 8);
        try {
            test.add(null, 10);
            fail();
        } catch (IllegalArgumentException ex) {}
    }