Django Serializer не является сериализуемым JSON

Я пытаюсь сериализовать детали объекта, который содержит ForeignKey и OneToOneField. Вот моя модель:

user = models.OneToOneField(
    "User",
    on_delete=models.CASCADE,
    null=False,
    blank=False,
    verbose_name="User",
    help_text="The user who subscribed.",
    related_name="subscription_information",
    unique=True,
)

subscription = models.ForeignKey(
    Subscription,
    on_delete=models.CASCADE,
    null=False,
    blank=False,
    related_name="subscription_information",
    verbose_name="Subscription",
    help_text="This is the subscription.",
)

subscription_type = models.IntegerField(
    choices=SUBSCRIPTION_TYPES_CHOICES,
    default=SubscriptionTypes.monthly,
    null=False,
    blank=False,
    verbose_name="Subscription Type",
    help_text="",
)

next_payment_amount = models.FloatField(
    default=0.0,
    null=False,
    blank=True,
    verbose_name="Subscription Plan Next Payment Amount",
    help_text=(""),
)

next_payment_date = models.DateTimeField(
    null=True,
    blank=True,
    default=None,
    verbose_name="Next Payment Date",
    help_text=(""),
)

payment_made = models.BooleanField(
    null=False,
    blank=True,
    default=False,
    verbose_name="Is Payment Made",
    help_text=(
        ""
    ),
)

subscription_date = models.DateTimeField(
    null=True,
    blank=True,
    default=None,
    verbose_name="Subscription Date",
    help_text="",
)

Как вы видите, поле User - это OneToOneField, а поле Subscription - foreign key.

А вот мой сериализатор:

class SubscriptionInformationDetailSerializer(serializers.ModelSerializer):

    class Meta:
        model = SubscriptionInformation
        fields = (
            "id",
            "user",
            "subscription",
            "subscription_type",
            "next_payment_amount",
            "next_payment_date",
            "payment_made",
            "subscription_date",
        )

Я хочу вернуть сериализованную SubscriptionInformation с помощью следующего кода:

subscription_information = SubscriptionInformation.objects.get(user_id=user.id) 
serializer = SubscriptionInformationDetailSerializer(subscription_information, read_only=True)

return serializer

Но он выдает эту ошибку:

Traceback (most recent call last):
  File "E:\Programming\project\venv\lib\site-packages\django\core\handlers\exception.py", line 42, in inner
    response = get_response(request)
  File "E:\Programming\project\venv\lib\site-packages\django\core\handlers\base.py", line 217, in _get_response       
    response = self.process_exception_by_middleware(e, request)
  File "E:\Programming\project\venv\lib\site-packages\django\core\handlers\base.py", line 215, in _get_response       
    response = response.render()
  File "E:\Programming\project\venv\lib\site-packages\django\template\response.py", line 109, in render
    self.content = self.rendered_content
  File "E:\Programming\project\venv\lib\site-packages\rest_framework\response.py", line 72, in rendered_content       
    ret = renderer.render(self.data, accepted_media_type, context)
  File "E:\Programming\project\venv\lib\site-packages\rest_framework\renderers.py", line 105, in render
    allow_nan=not self.strict, separators=separators
  File "E:\Programming\project\venv\lib\site-packages\rest_framework\utils\json.py", line 28, in dumps
    return json.dumps(*args, **kwargs)
  File "C:\Python27\Lib\json\__init__.py", line 251, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "C:\Python27\Lib\json\encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "C:\Python27\Lib\json\encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "E:\Programming\project\venv\lib\site-packages\rest_framework\utils\encoders.py", line 68, in default
    return super(JSONEncoder, self).default(obj)
  File "C:\Python27\Lib\json\encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: DetailSerializer(<Thing: Thing object>, read_only=True):      
    id = IntegerField(label='ID', read_only=True)
    user = PrimaryKeyRelatedField(help_text='The user who subscribed.', queryset=User.objects.all(), validators=[<UniqueValidator(queryset=SubscriptionInformation.objects.all())>])
    subscription = PrimaryKeyRelatedField(help_text='This is the subscription.', queryset=Subscription.objects.all())
    subscription_type = ChoiceField(choices=((0, 'Monthly'), (1, 'Annual')), help_text='', label='Subscription Type', required=False, validators=[<django.core.validators.MinValueValidator object>, <django.core.validators.MaxValueValidator object>])
    next_payment_amount = FloatField(help_text='', label='Subscription Plan Next Payment Amount', required=False)
    next_payment_date = DateTimeField(allow_null=True, help_text='', label='Next Payment Date', required=False)
    payment_made = BooleanField(help_text='', label='Is Payment Made', required=False)
    subscription_date = DateTimeField(allow_null=True, help_text='', label='Subscription Date', required=False) is not JSON serializable

Я не мог понять, почему я не могу сериализовать это. Почему это не JSON serializable

subscription_information = SubscriptionInformation.objects.get(user_id=user.id) 
serializer = SubscriptionInformationDetailSerializer(subscription_information, read_only=True)

return serializer.data