Aumentar la altura del keyboard personalizado iOS 8 no funciona

Intenté boost la altura del keyboard personalizado con el doc de Apple, pero no funciona. Aquí está mi código:

-(void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; CGFloat _expandedHeight = 500; NSLayoutConstraint *_heightConstraint = [NSLayoutConstraint constraintWithItem:self.view attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant: _expandedHeight]; [self.view addConstraint: _heightConstraint]; [super updateViewConstraints]; }