WrongValueException in Grid with paging mold

With the following ZUL code, I get a WrongValueException when pressing the "Clear" button:

<zk>
<grid mold="paging">
<rows id="theRows">
<group visible="false" label="Hidden group"></group>
<group label="Group" />
<row>
<button label="Clear" onClick="theRows.getChildren().clear()" />
</row>
</rows>
</grid>
</zk>

It only happens if the mold is paging and there is a hidden group. The behavior I expected was that this would work the same as it does in a different mold or without the hidden group, i.e. that no exception is thrown. If there is some error in the ZUL then I expected it to be reported in a less confusing and more descriptive way, and not be delayed until the call to clear().