public void testFormatAppend_PrinterParser_null_Parser() {
        PeriodParser parser = new PeriodFormatterBuilder().appendWeeks().appendLiteral("-").toParser();
        PeriodFormatterBuilder bld = new PeriodFormatterBuilder().append(null, parser).appendMonths();
        assertNull(bld.toPrinter());
        assertNotNull(bld.toParser());
        
        PeriodFormatter f = bld.toFormatter();
        try {
            f.print(PERIOD);
            fail();
        } catch (UnsupportedOperationException ex) {}
        assertEquals(new Period(0, 2, 1, 0, 0, 0, 0, 0), f.parsePeriod("1-2"));
    }