Columnlayout does not work with vflex

Test case:
<zk>
<div height="500px" style="border: 1px solid green">
<columnlayout vflex="1">
<columnchildren vflex="1" hflex="1">
<panel title="Panel" border="rounded+" vflex="1">
<panelchildren vflex="1">
Panel 1
</panelchildren>
</panel>
</columnchildren>
</columnlayout>
</div>
</zk>