Ios 隐藏admob中间广告后的空白区域？

在点击间隙广告的隐藏按钮后，我在屏幕底部得到了大约50px的空白，我无法检测到是什么导致了这一现象 这是我用来请求和查看插播广告的代码，有什么解决方法吗

    - (void)webViewDidFinishLoad:(UIWebView*)theWebView
{
    interstitial_ = [[GADInterstitial alloc] init];
    interstitial_.adUnitID = @"XXXXXXXXXXXXXXXXXXXXXX";
    interstitial_.delegate = self;
    GADRequest *request = [GADRequest request];
    request.testDevices = @[                         
                            GAD_SIMULATOR_ID,
                            ];

    [interstitial_ loadRequest:request];
    return [super webViewDidFinishLoad:theWebView];
}


- (void)interstitialDidReceiveAd:(GADBannerView *)adView {
    [interstitial_ presentFromRootViewController:self];
}