Unenveloping None values doesn't work as expected

test.py

#!/usr/bin/env python3

from marshmallow import Schema, fields, post_load, pre_load


class Resource(object):
    def __init__(self, *args, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)


class PersonSchema(Schema):
    id = fields.String()
    type = fields.String(required=True)
    address = fields.Nested('AddressSchema', allow_none=True)

    @pre_load(pass_many=True)
    def _unwrap_data(self, data, many):
        return data['data']

    @post_load
    def _make_object(self, data):
        return Resource(**data)


class AddressSchema(Schema):
    id = fields.String(required=True)
    type = fields.String(required=True)
    street = fields.String(required=True)
    city = fields.String(required=True)

    @pre_load(pass_many=True)
    def _unwrap_data(self, data, many):
        return data['data']

    @post_load
    def _make_object(self, data):
        return Resource(**data)


s_data = {'data': {'id': '5', 'type': 'employees', 'address': {'data': None}}}
schema = PersonSchema()
data, errors = schema.load(s_data)

print('data: {}'.format(data))
# {'id': '5', 'type': 'employees'}
print('errors: {}'.format(errors))
# {'address': {'id': ['Missing data for required field.'], 'city': ['Missing data for required field.'], 'type': ['Missing data for required field.'], 'street': ['Missing data for required field.']}}