def validate(self, value):
        """
        Validate the I{value} is of the correct class.
        @param value: The value to validate.
        @type value: any
        @raise AttributeError: When I{value} is invalid.
        """
        if value is None:
            return
        if len(self.classes) and not isinstance(value, self.classes):
            msg = '"%s" must be: %s' % (self.name, self.classes)
            raise AttributeError(msg)