Running NSTimer on a thread

I am trying to run a NSTimer on a thread using iPhone SDK 3.0. I think I am doing everything correctly (new runloop etc.). If I call [timer invalidate] on viewDidDissappear though I get this error: bool _WebTryThreadLock(bool), 0x3986d60: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now... Program received signal: “EXC_BAD_ACCESS”. Here is my code: ``` - (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; [activityIndicator startAnimating]; NSThread* timerThread = [[NSThread alloc] initWithTarget:self selector:@selector(timerStart) object:nil]; //Create a new thread [timerThread start]; //start the thread } -(void)timerStart { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; NSRunLoop* runLoop = [NSRunLoop currentRunLoop]; //Fire timer every second to updated countdown and date/time timer = [[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(method) userInfo:nil repeats:YES] retain]; [runLoop run]; [pool release]; } - (void)viewDidDisappear:(BOOL)animated { [super viewDidDisappear:animated]; [timer invalidate]; } ``` When I remove the line invalidating the timer everything works fine. Am I not supposed to invalidate it or am I making some other mistake? Thanks