Setting Empty on an Entity field fails

[six8]

Setting Empty is allowed, but set tries to return the value that no longer exists.

    def set(self, instance, name, value):
        if value is Empty:
            if name in instance.__values__:
                del instance.__values__[name]
        else:
            instance.__values__[name] = self.adapt(value)
>       return instance.__values__[name]