def test_invalid_override(self):
        Decimal = C.Decimal

        try:
            from locale import CHAR_MAX
        except ImportError:
            self.skipTest('locale.CHAR_MAX not available')

        def make_grouping(lst):
            return ''.join([chr(x) for x in lst])

        def get_fmt(x, override=None, fmt='n'):
            return Decimal(x).__format__(fmt, override)

        invalid_grouping = {
            'decimal_point' : ',',
            'grouping' : make_grouping([255, 255, 0]),
            'thousands_sep' : ','
        }
        invalid_dot = {
            'decimal_point' : 'xxxxx',
            'grouping' : make_grouping([3, 3, 0]),
            'thousands_sep' : ','
        }
        invalid_sep = {
            'decimal_point' : '.',
            'grouping' : make_grouping([3, 3, 0]),
            'thousands_sep' : 'yyyyy'
        }

        if CHAR_MAX == 127: # negative grouping in override
            self.assertRaises(ValueError, get_fmt, 12345,
                              invalid_grouping, 'g')

        self.assertRaises(ValueError, get_fmt, 12345, invalid_dot, 'g')
        self.assertRaises(ValueError, get_fmt, 12345, invalid_sep, 'g')