NSURLSession : getting null resumedata when downloading is running and in between that internet connection will change as ON to OFF mode

Question

I am using NSURLSession for downloading zip file from server. following task working perfectly

• Pause and resume downloading
• Get Progress of downloading
• Get Notification of when internet is active or not active

but there is major problem

While downloading is going on at that time if internet connection is gone that time I get notification about it but there I can stored NSData which I will again assign when internet connection will come. In internet connective method I get null data from

[downloadTask cancelByProducingResumeData:^(NsData * resumeData){
NSLog(@"Resume data %@",resumeData);  // GETTING **"NULL"**

}

Please help!