Error while using NSUserDefaults -

i have app trying call method once no matter how many times user runs app. right using this:

nsuserdefaults *defaults = [nsuserdefaults standarduserdefaults]; if ([defaults objectforkey:@"runonce"]==nil)     {        //my method         [defaults setobject:@"yes" forkey:@"runonce"];         [defaults synchronize];    } 

but when run code error:

index 0 beyond bounds empty array

this first time using nsuserdefaults i'm not sure i'm doing wrong.

this problem applied database, not particular one.