iOS Help! admob Interstitial Ads giving error "No UIViewController supplied to the ad"

[Manesh]

The first interstitial ad loads fine,
but the 2nd and onwards gives the following error in my NSLOG

<Google> No UIViewController supplied to the ad. Cannot continue.

In my GameOver void, it says the below :

if ([self.interstitial isReady]) {
        self.interstitial.delegate = self;
        [self.interstitial presentFromRootViewController:self.viewController];
    }

any idea whats happening here?
why is it not picking up the self.viewcontroller?
thanks