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

i using nsurlsession downloading zip file server. following task working perfectly

• pause , resume downloading
• get progress of downloading
• get notification of when internet active or not active

but there major problem

while downloading going on @ time if internet connection gone time notification there can stored nsdata again assign when internet connection come. in internet connective method null data from

[downloadtask cancelbyproducingresumedata:^(nsdata * resumedata){ nslog(@"resume data %@",resumedata);  // getting **"null"**  } 

please help!