ios7+ how to add contrainsts to uiimagepickercontroller cameraOverlay

Question

I'm quite surprised. I didn't find anything remotely tackling the issue on SO. I would have imagined that many people may have tried that before me. That must only mean one thing ... It's trivial and I can't make it :(

How do I add constraints to the cameraOverlay ?

I tried this

UIView *overlayView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, screenWidth, screenHeight)];
    [overlayView setBackgroundColor:[UIColor redColor]];
    [overlayView setAlpha:0.5];
    UILabel *Testlabel = [[UILabel alloc] init];
    [Testlabel setText:@"Hollllaaaaa in the overlay"];
    [overlayView addSubview:Testlabel];

    NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:overlayView
                                                                       attribute:NSLayoutAttributeCenterX
                                                                       relatedBy:NSLayoutRelationEqual
                                                                          toItem:Testlabel
                                                                       attribute:NSLayoutAttributeCenterX
                                                                      multiplier:1
                                                                        constant:0];

    [overlayView addConstraints:@[widthConstraint]];

    [self.picker setCameraOverlayView:overlayView];

But I get the error Unable to simultaneously satisfy constraints. With just one constraint.

As a note, the picker is part of a view that has constraints and are well defined. I am wondering if the fact that I initwithFrame the overlay view has something to do with autolayout not liking that ... If that the case, how can I simply add a constraint to a blank overlay that would have the same size as the screen ?