@Override
    public String convertTo(Set<IpSubnet> value) {
        if (value == null) {
            throw new ParameterException("Couldn't convert IP subnets <null> to string.");
        }
        return Joiner.on(",").skipNulls().join(value);
    }
@Test
    public void convertToThrowsParameterExceptionWithNull() {
        expectedException.expect(ParameterException.class);
        expectedException.expectMessage("Couldn't convert IP subnets <null> to string.");
        converter.convertTo(null);
    }