Verifying purchase receipts in both iOS 6 and 7

I am verifying in-app-purchase receipts using the following code:

- (void) completeTransaction:(SKPaymentTransaction*) transaction
{   
    NSData* receipt = nil;

    // 1. Attempt <app receipt> first (iOS 7.x)

    NSBundle* mainBundle = [NSBundle mainBundle];

    if ([mainBundle respondsToSelector:@selector(appStoreReceiptURL)]) {

        NSURL* appStoreReceiptURL = [mainBundle appStoreReceiptURL]; // <- CRASH

        receipt = [NSData dataWithContentsOfURL:appStoreReceiptURL];
    }

    // 2. Fallback to <transaction receipt> (iOS < 7.0)
    if (!receipt) {    
        receipt = [transaction transactionReceipt];
    }

    // 3. Have server verify it with iTunes:    
    [self verifyReceipt:receipt forTransaction:transaction];
}

On an iOS 6 device, the execution stops at the line NSURL* appStoreReceiptURL = [mainBundle appStoreReceiptURL]; and the console spits:

-[NSBundle appStoreReceiptURL]: unrecognized selector sent to instance 0x208492d0

Am I missing something? Wasn't -respondsToSelector: supposed to take care of this? Must I fall back to checking the OS version directly??