unable to resolve class org.zkoss.zk.ui.util.Clients.

[mauro]

I've create a new grails app with zkui plugin.
Grails version is 2.3.1.
Create the domain classes and do zkui-generate-all.
When I open the edit page for a domain class I have this error:

Script1.groovy: 3: unable to resolve class org.zkoss.zk.ui.util.Clients
@ line 3, column 2.
import org.zkoss.zk.ui.util.Clients

I don't understand...I've searched the import under all of my files but I didn't find it.
Can you give a help?

[mauro]

The problem is this select generated by zkui-generate-all:

<zkui:select name="sector.id" from="${zktest.Sector.list()}" optionKey="id" value="${categoryInstance?.sector?.id}"  />

what's the problem?