NSFetchRequest and predicateWithBlock

I am playing with an app that uses Core Data and NSManagedObjects to populate a UITableView. There is only one class in my application, called Event. I have created the following custom instance method on Event:

- (BOOL)isExpired {
    return ([[self.endOn dateAtEndOfDay] timeIntervalSinceNow] < 0);
}

I would like to limit the UITableView that displays Event objects to only the Events that are expired - that is, where isExpired returns YES. I have tried to do this by adding an NSPredicate to the NSFetchRequest:

NSPredicate *predicate = [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary * bindings) {return([evaluatedObject isExpired]);}];
[fetchRequest setPredicate:predicate];

but I get the error: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Problem with subpredicate BLOCKPREDICATE(0x272ac)' ***. Does this mean that you can't use a block predicate with an NSFetchRequest? Or have I just constructed it improperly?

Thank you!