Admob ads not appear in footer of UITableView when keyboard is shown

I use this to show Admob ads on the footer of UITableView:

- (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section {
    GADBannerView *sampleView = [[GADBannerView alloc] initWithAdSize:kGADAdSizeBanner];

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
    {
        sampleView.hidden=true;
    }

    sampleView.adUnitID = @"myID";

    sampleView.rootViewController = self;

    [sampleView loadRequest:[GADRequest request]];
    return sampleView;
}
- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section {
    return 50.0;
}

I do not have real iPhone device, so I only test it on simulators. The above code works if the software keyboard is hidden and I typed the word using my MacBook keyboard. However, when I open the software keyboard in simulator, the ads failed to load at footer. Instead it loads directed below the searching bar. How should I solve this? I don't know if this bug also occurred in the real device.