Oh, to_dict also needed to be updated with the

if type

in the first dict composition:

def to_dict(self):
        '''Return the raw JSON Schema as a (possibly nested) dict.'''
        result = {}

        if self.properties is not None:
            result['properties'] = {
                k: v.to_dict() if type(v) is Schema else v
                for k, v
                in self.properties.items()  # pylint: disable=no-member
            }


        if self.items is not None:
            result['items'] = self.items.to_dict()  # pylint: disable=no-member

        for key in STANDARD_KEYS:
            if self.__dict__.get(key) is not None:
                result[key] = self.__dict__[key]

        return result