@synchronized doesn't retain monitored object under MRC

gistfile1.m

    __block id foo = [MyObject new];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"block entering synchronized");
        @synchronized(foo) {
            NSLog(@"running in synchronized");
            sleep(10);
            // what does @synchronized do the now-zombie foo as part of its closure?
        }
        NSLog(@"block exited synchronized");
    });
    sleep(1);
    NSLog(@"running at top level, releasing foo");
    [foo release];
    NSLog(@"running at top level, released foo");