def METHOD_NAME(polymorphic_dict):
    # Test case to ensure no further regressions on issue #359
    polymorphic_dict['definitions']['GenericPet']['x-nullable'] = True
    polymorphic_spec = Spec.from_dict(polymorphic_dict)
    PetList = polymorphic_spec.definitions['PetList']
    response = unmarshal_response(
        response=Mock(
            spec=IncomingResponse,
            status_code=200,
            headers={'content-type': APP_JSON},
            json=Mock(
                return_value={
                    'number_of_pets': 1,
                    'list': [None],
                },
            ),
        ),
        op=polymorphic_spec.resources['pets'].operations['get_pets'],
    )
    assert response == PetList(number_of_pets=1, list=[None])