iphone - Rate the app functionality in ios not working well

Question

I am having an app in which I have a button to "Rate" the app.

On click of the button, I am using the following code.

- (IBAction)ratePressed:(id)sender
{


    NSString *str = @"itms-apps://ax.itunes.apple.com/WebObjects/MZStore.woa";
    str = [NSString stringWithFormat:@"%@/wa/viewContentsUserReviews?", str];
    str = [NSString stringWithFormat:@"%@type=Purple+Software&id=", str];

    // Here is the app id from itunesconnect 
    str = [NSString stringWithFormat:@"%@appid", str];
    NSLog(@"str is %@",str);

    [[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];

}

My App is not on the app store so when i Pressed the rate button it gives me an error saying

Your request produced an error. [new NullResponse]

When I put the appid of of an app that is actually on the appstore. It works fine.

So can I put the custom message saying "app is currently not available"? when the app is not on the appstore. Or is it a default message by Apple?

Please help me.