    @Test
    public void testFromHexStringError() {

        String invalidStr = "00:00:00:00:00:00:ffff";
        try {
            HexString.fromHexString(invalidStr);
            fail("HexString.fromHexString() should have thrown a NumberFormatException");
        }
        catch (NumberFormatException expected) {
            // do nothing
        }
    }