zk5 and jsf problem editor

<? taglib uri="http://www.zkoss.org/jsf/zul" prefix="z"?>
<?page title="new page title" contentType="text/html;charset=UTF-8"?>
<zk>
<z:window title="new page title" border="normal">
	New Content Here!
	<calendar></calendar>
</window>
</zk>  

The editor fire the error: "Fail to render : org.xml.sax.SAXParseException: The prefix "z" for element "z:window" is not bound.".

What is this?