Centre image in a div with flexbox

gistfile1.txt

http://jsfiddle.net/danield770/Ld8bcc0p/1/

div.do_not_define_height {
    display: flex;
    justify-content: center;
    /* align horizontal */
    align-items: center;
    /* align vertical */
    width: 100%;
    height: 100%;    
}

img {
  width: 80%;
  height: auto;
}

<div style="border: 1px solid green; width: 80vw; height: 80vw;">
<div class="do_not_define_height">
    <img alt="No, he'll be an engineer." src="http://placehold.it/350x150" />
</div>
</div>