The problem with this argument is that in the modern liberal order (and again ignoring some marginal exceptions), the state has a monopoly of violence, including violence that may be necessary to enforce a contract. Therefore, the state not only refuses to apply violence to enforce your claim based on such a contract, but will also intervene violently to stop you if you try to enforce it with private force. It is a criminal offence to breach the peace even in the course of privately enforcing a valid contractual claim, let alone one that is legally declared void.

Germany reportedly enforces illegal contracts.