Borderlayout North/South failed to concern caption height

<zk>
<borderlayout>
<north title="North" vflex="min">
<div style="border: 1px solid green">
North
</div>
</north>
<south title="South" vflex="min">
<div style="border: 1px solid green">
South
</div>
</south>
<center>
<div style="border: 1px solid blue">
Center
</div>
</center>
</borderlayout>
</zk>