What is the ruling on bequeathing the share of an heir who has no inheritable share?

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If a person bequeaths the share of someone who legally has no inheritance share—such as bequeathing the share of a son who is a slave or belongs to a different religion, or bequeathing the share of a brother who is legally veiled (mahjub) from inheriting—the legatee receives nothing. This is because the specified share upon which the bequest is based is non-existent in reality, and therefore its like is nothing.