public URL convert(String value) {
        if (isBlank(value)) {
            throw new ParameterException(getErrorString("a blank value", "a valid URL"));
        }

        try {
            return URLUtil.parseURL(value);
        } catch (IllegalArgumentException e) {
            throw new ParameterException(getErrorString(value, "a valid URL"));
        }

    }
@Test(expected = ParameterException.class)
    public void nullValueThrowsParameterException() {
        converter.convert(null);
    }