Modal transition via presentViewController:animated ignores the animation

Modal transition via presentViewController:animated ignores the animation

I am trying to display a QLpreviewcontroller on a UIVIewcontroller.
-(void)showPreview{
if(!_previewController){
_previewController = [[QLPreviewController alloc] init];
_previewController.dataSource = self;
_previewController.delegate = self;
}
// There will always be one item here
_previewController.currentPreviewItemIndex = 0;
[_previewController
setModalTransitionStyle:UIModalTransitionStyleCoverVertical];
[self presentViewController:_previewController animated:YES
completion:nil];
}
the previecontroller is displayed but the transition is not animated. it
just appears without any animation.