Hello, my app crashes often in this for-loop:

for (int a = 0; a <= 20; a++) {
        int b = arc4random() % [newsStories count];
        NSString * foo = [[NSString alloc]initWithString:[[newsStories objectAtIndex: arc4random() % b]objectForKey:@"title"]];
        foo = [foo stringByReplacingOccurrencesOfString:@"’" withString:@""];
        foo = [[foo componentsSeparatedByCharactersInSet:[[NSCharacterSet letterCharacterSet] invertedSet]] componentsJoinedByString:@" "];
        foo = [foo stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
        textView.text = [textView.text stringByAppendingString:[NSString stringWithFormat:@"%@ ", foo]];
    }

The code changes a NSString from a NSDictionary and adds it into a textView. Sometimes it first crashes in the second time using the for-loop. What did I wrong?