IE 11 flexbox with flex-wrap: wrap doesn't seem to calculate widths correctly (box-sizing ignored?)

Seems IE11 doesn't calculate flex item widths properly if flex-wrap: wrap is used.

See http://jsfiddle.net/MartijnR/WRn9r/20/

The 4 boxes each have flex-basis: 50% so we should get two lines of two boxes each, but in IE11 each box gets one line. When setting the border-width to 0, it works correctly.

Is this is a bug, and/or is there is way to make IE11 behave (and still use borders)?

<section>
    <div class="box">1</div>
    <div class="box">2</div>
    <div class="box">3</div>
    <div class="box">4</div>
    <div class="box">5</div>
</section>
section {
    display: -moz-webkit-flex;
    display: -webkit-flex;
    display: flex;
    -ms-flex-direction: row;
    -moz-flex-direction: row;
    -webkit-flex-direction: row;
    flex-direction: row;
    -ms-flex-wrap: wrap;
    -moz-flex-wrap: wrap;
    -webkit-flex-wrap: wrap;
    flex-wrap: wrap;
    width: 100%;
    box-sizing:border-box;
    margin:0;
}
.box {
    border: 1px solid black;
    background: #ccc;
    display: block;
    -ms-flex: 50%;
    -moz-flex: 50%;
    -webkit-flex: 50%;
    min-width: 50%;
    flex: 50%;
    box-sizing: border-box;
    margin:0;
}