How to get rid of the 'undeclared selector' warning

I want to use a selector on an NSObject instance without the need for an implemented protocol. For example, there's a category method that should set an error property if the NSObject instance it's called on supports it. This is the code, and the code works as intended:

if ([self respondsToSelector:@selector(setError:)])
{
    [self performSelector:@selector(setError:) withObject:[NSError errorWithDomain:@"SomeDomain" code:1 userInfo:nil]];
}

However, the compiler doesn't see any method around with the setError: signature, so it gives me a warning, for each line that contains the @selector(setError:) snippet:

Undeclared selector 'setError:'

I don't want to have to declare a protocol to get rid of this warning, because I don't want all classes that may use this to implement anything special. Just by convention I want them to have a setError: method or property.

Is this doable? How?