The formation of contract is done willingly : no duress, no fraud, and no mistake.
Normally the parties will indicate in the contract that if the terms or conditions in the contract become invalid, unenforceable, or null and void, it will not affect other conditions in the contract which is still valid.
E.g. Should any of the clauses hereunder becomes invalid or unenforceable for any reason whatsoever, such invalidity or unenforceability shall not affect other clauses which shall remain in full force and effect.