out of scope exception

Hello all.
I have some code which read buffer and write this value in variable, but on the second iteration of timer i get out of memory exception, what is mean?
Thanks.

- (void)applicationDidFinishLaunchingNSNotification *)aNotification {
self.previousString == [NSString stringWithString:[self stringFromPasteboard]];

NSLog(@"%@", self.previousString);

[self startTimer];
}

- (void)startTimer {
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selectorselector(readPasteboard userInfo:nil repeats:YES];
}

- (void)readPasteboardNSTimer *)theTimer {
NSLog(@"%@", self.previousString); //<- this i hawe mistake.
}

- (NSString *)stringFromPasteboard {
NSPasteboard *pasteboard = [NSPasteboard generalPasteboard];

NSArray *classes = [[NSArray alloc] initWithObjects:[NSString class], nil];

NSDictionary *options = [NSDictionary dictionary];

NSArray *copiedItems = [pasteboard readObjectsForClasses:classes optionsptions];

return [copiedItems objectAtIndex:0];
}