listbox with checkmark="true" multiple="true" however the multiple not working, it is now showing only single selection

Here's the existing code that works before ZK 6

in zul page :

<listbox id="lst" checkmark="true" multiple="true" ctrlKeys="^a" droppable="true" >

in JAVA class :

lst.setModel(new ListModelList(List<String> data));

ListitemRenderer lstItem = new ListitemRenderer() {

@Override
public void render(Listitem lstm, Object o, int i) throws Exception

{ lstm.setLabel(o.toString()); lstm.setValue(o.toString()); lstm.setDraggable("true"); }

};

lst.setItemRenderer(lstItem);