Why listbox checkmark=true shows me radiobutton and not checkbox ??

<zk xmlns="http://www.zkoss.org/2005/zul">
    <window id="permisoWIN" title="Asignar permiso" closable="true" 
    apply="pbz.ctrl.man.win.PermisoWIN"
    border="normal" use="pbz.ctrl.ext.MyWindow" width="630px" height="390px">
         <toolbar>
            <toolbarbutton id="btnSave"   label="Guardar" image="/ico/Save24.png" />
            <toolbarbutton id="btnCancel" label="Cancelar" image="/ico/Cancel24.png"/>      
        </toolbar>
        <div style="padding:1% 0% 2% 0%;">
      <listbox id="listbox" checkmark="true" multiple="true" height="300px" >
        <listhead>
          <listheader label=" " width="60px"/>
          <listheader label="Formulario" width="270px"/>
          <listheader label="Grupo" width="260px"/>
        </listhead>     
      </listbox>
    </div>
    </window>
</zk>