NSManagedObjectContext performBlockAndWait: doesn't execute on background thread?

I have an NSManagedObjectContext declared like so:

- (NSManagedObjectContext *) backgroundMOC {
    if (backgroundMOC != nil) {
        return backgroundMOC;
    }
    backgroundMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    return backgroundMOC;
}

Notice that it is declared with a private queue concurrency type, so its tasks should be run on a background thread. I have the following code:

-(void)testThreading
{
    /* ok */
    [self.backgroundMOC performBlock:^{
        assert(![NSThread isMainThread]); 
    }];

    /* CRASH */
    [self.backgroundMOC performBlockAndWait:^{
        assert(![NSThread isMainThread]); 
    }];
}

Why does calling performBlockAndWait execute the task on the main thread rather than background thread?