How does an Objective-C property setter signal failure?

Suppose you have a property with copy semantics. What should you do in the setter if the copy method fails? (I presume this is a possibility, since a copy usually starts with an alloc/init combo, which can fail and return nil.) Apple recommends returning error codes rather than using exceptions, but a setter generally has a void return type. What is the recommended approach? How do you signal that an error has occurred?