Unsupported child for columns with foot : <Label null>

<grid>
<columns children="@load(vm.columns)" sizable="true">
<template name="children" var="col">
<column label="${col.title}"/>
</template>
<columns>
<foot children="@load(vm.columns)">
<template name="children" var="col">
<footer>
<label value="${col.xxx}"/>
</footer>
</template>
</foot>
</grid>

this is valid before zk version 6.5.3