public void testFormatAppend_PrinterParser_Printer_null() {
        PeriodPrinter printer = new PeriodFormatterBuilder().appendYears().appendLiteral("-").toPrinter();
        PeriodFormatterBuilder bld = new PeriodFormatterBuilder().append(printer, null).appendMonths();
        assertNotNull(bld.toPrinter());
        assertNull(bld.toParser());
        
        PeriodFormatter f = bld.toFormatter();
        assertEquals("1-2", f.print(PERIOD));
        try {
            f.parsePeriod("1-2");
            fail();
        } catch (UnsupportedOperationException ex) {}
    }