Django Serializer field validator not being called

Question

Serializer field validator not being called seems to be a common problem but I cannot find the right solution. I have a normal (NON-MODEL) serializer with a URLField and a custom validator for the field. The field validator is not being hit when running is_valid(), instead the builtin URLValidator is being called.

Here is the custom validator (only to avoid http:// not included error):

class OptionalSchemeURLValidator(URLValidator):
    def __call__(self, value):
        if "://" not in value:
            value = "http://" + value
        super(OptionalSchemeURLValidator, self).__call__(value)

Here is the serializer:

from rest_framework.serializers import Serializer
class DeveloperAccessTokenSerializer(Serializer):
    token = CharField(read_only=True)
    company_website = URLField(
        required=False,
        validators=[OptionalSchemeURLValidator()],
    )

    def create(self, validated_data):
        self.token = jwt.encode(
            jwt_payload, settings.SECRET_KEY, algorithm="HS256"
        )
        self.company_website = validated_data.get("company_website", None)
        return self.company_website

Here is how the serializer is being used:

def post(self, request, *args, **kwargs):
    context = {"request": request}
    ser = self.serializer_class(data=request.data, context=context)
    ser.is_valid(raise_exception=True)
    token = ser.save()
    return Response(token, status=HTTP_201_CREATED)

Validation error is being raised on is_valid():

rest_framework.exceptions.ValidationError: {'company_website': [ErrorDetail(string='Enter a valid URL.', code='invalid')]}

Input value for the field is

www.my-website.com