Symfony将日期序列化为空数组

use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

class DateTimeNormalizer implements NormalizerInterface
{
    public function normalize($object, $format = null, array $context = [])
    {
        return $object->format(\DateTime::ATOM);
    }

    public function supportsNormalization($data, $format = null)
    {
        return $data instanceof \DateTime;
    }
}

# config/services.yaml
services:
    App\Normalizer\DateTimeNormalizer:
        tags: [serializer.normalizer]

use Symfony\Component\Serializer\Serializer;

$serializer = new Serializer([new DateTimeNormalizer()], ['json' => new JsonEncoder()]);

$date = new \DateTime();
$serializedDate = $serializer->normalize($date);