iphone: NSPredicate problem with NSFetchedResultsController

Question

I currently have a 'Topic' entity defined in my system as:

@interface Topic :  NSManagedObject  
{
}
@property (nonatomic, retain) NSString * path;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * topicID;
@property (nonatomic, retain) NSNumber * parent;
@end

I want to fetch a topic with a specific number (e.g. 4001) using an NSFetchedResultsController. I've defined mine as:

- (NSFetchedResultsController *)fetchedResultsController {

    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Topic" inManagedObjectContext:_context];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"topicID == 4100"];
    [fetchRequest setPredicate:predicate];


    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"path" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    [fetchRequest setFetchBatchSize:20];

    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:@"Root"];
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;

    [fetchRequest release];
    [theFetchedResultsController release];

    return _fetchedResultsController;    

}

..Without the predicate, the FetchedResultsController works fine.. however, the moment I add it, there is an error and the app crashes.. can someone please tell me if I've defined my NSPredicate incorrectly?