FlexBox making content disappear



I have a modal that displays some images. I want theses images to be centered in the middle of the screen.
However when i set the display of the parent to flex, the image disappears.
Any idea why ?

<div on-swipe-up="closeModal('imageviewer')">   

        <ion-slide-box on-slide-changed="slideChanged(index)" active-slide="modal[1]">
            <ion-slide ng-repeat="imgUrl in modal[2]">
                /* Image Display */


The problem occurs when i add display: flex; to the parent div. If you guys have any explanation or alternative solution, i’ll be thankfull