NSFetchedResultsController does not find any data, but normal NSFetchRequest does

Question

I am new to Core Data and got really confused about this problem. I use an NSFetchedResultsController to get my data from the Core Data storage. After trying to delete data, my app crashes and the FRC doesn't work anymore. I deleted all data and tried it again, but it still doesn't work.

The confusing thing is that a standard NSFetchRequest works.

Here is my code for the FRC:

(NSFetchedResultsController *)fetchedResultsController {
if (fetchedResultsController != nil) {
    return fetchedResultsController;
}

StundenZettelAppDelegate *delegate = [[UIApplication sharedApplication] delegate];
managedObjectContext = delegate.managedObjectContext;

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = 
[NSEntityDescription entityForName:@"Employer"
            inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] 
                                    initWithKey:@"name" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

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

//TEST to see if it works, both values are 0
int i = [[aFetchedResultsController sections] count];
int j = [[[aFetchedResultsController sections] objectAtIndex:0] numberOfObjects];


[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];

return fetchedResultsController;

}

Here is a code example for the NSFetchRequest which works. (I used the code from the apple tutorial):

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Employer" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];

NSError *error;
NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
if (mutableFetchResults == nil) {
    // Handle the error.
}

Does anyone have any idea why this does not work?