@ -143,8 +143,10 @@ abstract class BaseResponse extends Response {
$writer->startElement($k);
$this->toXML($v->jsonSerialize(), $writer);
$writer->endElement();
} elseif ($v === null) {
$writer->writeElement($k);
} else {
$writer->writeElement($k, $v);
$writer->writeElement($k, (string)$v);
}