ios - Receive null device token sometimes -

i'm working on project allows user receive push notifications whenever there new on user account. i'm using parse push notifications service. i'm having no problem until our server starting receive empty token device on every push notification registration, problem not happening. when tried app on device run should when app tested on our client device , our server receive empty token device client user. how can happen? how can fix this? , how best practice , set device token?

here code in appdelegate:

- (void) application:(uiapplication *)application didregisterforremotenotificationswithdevicetoken:(nsdata *)devicetoken{ const unsigned *tokenbytes = [devicetoken bytes]; nsstring *token = [nsstring stringwithformat:@"%08x%08x%08x%08x%08x%08x%08x%08x",                             ntohl(tokenbytes[0]), ntohl(tokenbytes[1]), ntohl(tokenbytes[2]),                             ntohl(tokenbytes[3]), ntohl(tokenbytes[4]), ntohl(tokenbytes[5]),                             ntohl(tokenbytes[6]), ntohl(tokenbytes[7])];  //function saving device token server [[asengine defaultengine] setcurrentdevicetoken:token]; if([[asengine defaultengine] currentcredential] != nil) {     [[asengine defaultengine] webstoredevicetoken:token];  }  //save current instalation parse pfinstallation *currentinstallation = [pfinstallation currentinstallation]; [currentinstallation setdevicetokenfromdata:devicetoken]; [currentinstallation saveinbackgroundwithblock:^(bool succeeded, nserror *error) {     nslog(@"error e current installation: %@", error); }];  //save device token locally [[nsuserdefaults standarduserdefaults] setobject:devicetoken forkey:@"devicetoken"]; } 

an empty token wont ever generated, ios provides didfailtoregisterforremotenotificationswitherror method being called in case, please make sure check errors in token creation.