In App Purchase Receipt verification within app

I want to verify the transaction receipt within my app,

Here is my code,

- (void)recordTransaction:(SKPaymentTransaction *)transaction {

    NSData *receiptData = [NSData dataWithData:transaction.transactionReceipt];

    NSString *encodedString = [Base64 encode:receiptData];

     NSURL *url = [NSURL URLWithString:@"https://sandbox.itunes.apple.com/verifyReceipt"];

    ASIFormDataRequest *request = [[ASIFormDataRequest alloc] initWithURL:url];

    [request setPostValue:encodedString forKey:@"receipt-data"];

    [request setRequestMethod:@"POST"];

    [request setDelegate:self];

    [request startAsynchronous];

}

I am getting output:

{"status":21002, "exception":"java.lang.NullPointerException"}

Can someone help me to get proper receipt verification?