public void testForFields6() throws Exception {
        DurationFieldType[] types = null;
        try {
            PeriodType.forFields(types);
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
        
        types = new DurationFieldType[0];
        try {
            PeriodType.forFields(types);
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
        
        types = new DurationFieldType[] {
            null,
            DurationFieldType.months(),
        };
        try {
            PeriodType.forFields(types);
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
        
        types = new DurationFieldType[] {
            DurationFieldType.months(),
            null,
        };
        try {
            PeriodType.forFields(types);
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
    }