Messagebox get error when set the Div be space owner

<zk>
<zscript><![CDATA[
class MyDiv extends org.zkoss.zul.Div implements IdSpace {
}
]]></zscript>
<div use="MyDiv"/>
<button label="Click me twice, it will show error" width="150px">
<attribute name="onClick">
Messagebox.show("Question is pressed. Are you sure?", "Question", Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION);
</attribute>
</button>
</zk>