IOS编程报错问题，请指教，急！！！

*** Terminating app due to uncaught exception ✀NSRangeException✀, reason: ✀-[__NSCFConstantString substringWithRange:]: Range {2147483648, 4294967295} out of bounds; string length 0✀ 报错，请问为何？源代码如下： [mgr GET:@"https://api.weibo.com/2/statuses/home_timeline.json" parameters:params success:^(AFHTTPRequestOperation *operation, id responseObject) {NSArray *statusArray = [YSStatus objectArrayWithKeyValuesArray:responseObject[@"statuses"]];NSMutableArray *statusFrameArray = [NSMutableArray array]; for (YSStatus *status in statusArray) {YSStatusFrame *statusFrame = [[YSStatusFrame alloc]init];statusFrame.status = status;[statusFrameArray addObject:statusFrame];}