Grid hflex fail while the label in a div with FF4

<zk>
<grid>
<columns>
<column hflex="min" label="1" />
</columns>
<rows>
<row>
<div hflex="min">
<label value="Appointment" />
</div>
</row>
<row>
<div hflex="min">
<label value="Appointment" />
</div>
</row>
</rows>
</grid>
</zk>