Saving NSMutableArray to NSUserDefaults

I have an NSMutableArray inside of my AppDelegate, and I'm attempting to save it to NSUserDefaults. The NSMutableArray (strainsfinal) contains a list of favorited items. I want this list to save, and be present even if the app is shut down, and then restarted. Does anyone know why my code isn't working (not saving my data)? See below: AppDelegate.m -(void)updateStrains:(NSDictionary *)item { NSUserDefaults *currentDefaults = [NSUserDefaults standardUserDefaults]; NSData *dataSave = [currentDefaults objectForKey:@"strains"]; if (strainsfinal != nil) { NSArray *oldSavedArray = [NSKeyedUnarchiver unarchiveObjectWithData:dataSave]; if (oldSavedArray != nil) strainsfinal = [[NSMutableArray alloc] initWithArray:oldSavedArray]; else strainsfinal = [[NSMutableArray alloc] init]; } } EDIT: Appdelegate.m So I've changed my above method to the method below (and I thought it should work), but the app crashes, saying: "Terminating app due to uncaught exception 'NSInvalidArguementException', reason: 'NSConcreteAttributedString initwithString: nil 'value'. What am I missing?! -(void)updateStrains:(NSDictionary *)item { NSData *dataSave = [NSKeyedArchiver archivedDataWithRootObject:strainsfinal]; [[NSUserDefaults standardUserDefaults] setObject:dataSave forKey:@"strains"]; NSNotificationCenter* nc = [NSNotificationCenter defaultCenter]; [nc postNotificationName:@"arrayupdated" object:self userInfo:nil]; NSLog(@"updated strains %@",strainsfinal); }