CoreData asynchronous fetch causes concurrency debugger error

I'm using the

-com.apple.CoreData.ConcurrencyDebug

argument on launch to debug concurrency in my CoreData app.

During app launch, I perform an asynchronous fetch on the main thread's managed object context.

// set up the async request
   NSError * error = nil;
        [MOC executeRequest:asyncFetch error:&error];
        if (error) {
            NSLog(@"Unable to execute fetch request.");
            NSLog(@"%@, %@", error, error.localizedDescription);
        }

This code is called from the main thread, but executeRequest: enqueues it to another thread, which I understand to be the correct behavior.

The concurrency debugger doesn't like this, saying (I reckon) that I'm doing something wrong here. I've also tried wrapping this in [MOC performBlock:] which also works, but also causes a multithreading violation. In both cases I get this :

[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__

Am I using async fetches incorrectly, or is the concurrency debugger wrong here?

EDIT : I've also tried wrapping it in MOC performBlock which should ensure that it gets called from the main thread. In any case, the call is enqueued from the main thread, but executed elsewhere.