UIAlertView装载指示器几乎越位？

I have following code:

我有以下代码:

_loadingAlert = [[UIAlertView alloc] initWithTitle:ALERT_TITLE_LOADING
                                           message:ALERT_MESSAGE_LOADING
                                          delegate:nil
                                 cancelButtonTitle:nil
                                 otherButtonTitles:nil];

UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
[indicator startAnimating];

[_loadingAlert setValue:indicator forKey:@"accessoryView"];
[_loadingAlert show];
_loadingAler