When user removes "Works with Nest" for my client, the "withCancelBlock" is not called

Question

I'm using the following code to get notified when the user revokes permission from my client. Specifically, I expect the withCanelBlock to be called when the user revokes permission, but this block is never called.

        [newFirebase observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
        [self.subscribedURLs setObject:snapshot forKey:URL];
        NSLog(@"%@", snapshot.value);
        block(snapshot);
    } withCancelBlock:^(NSError *error) {
        NSLog(@"%s: permission revoked!!!!!", __PRETTY_FUNCTION__);
    }
     ];