iOS: Why do I have white, rounded corners on my modal view?

I have a modal view popping up in my iPad app and for some reason it has white, rounded corners.

It might be worth noting I built this model view in my storyboard, not programmatically. However, in my viewWillAppear method, I'm setting the corner radius like so...

- (void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    self.view.layer.cornerRadius = 6.0f;
}

When I set the value above 6, the white corners become visible. How can I set the value higher without these white rounded corners showing?

Thanks so much in advance for your wisdom!