NSURLConnection sendAsynchronousRequest：永远不会调用块

Question

程序流永远不会通过NSLog-Message到达Block。我在这里错过了什么吗？

[NSURLConnection 
    sendAsynchronousRequest:req  
    queue: [NSOperationQueue mainQueue] 
    completionHandler: ^(NSURLResponse *response_, NSData *data_, NSError *error_) {
         NSLog(@"Why am I not being called %@", data_);
    }];