MVVM form can't take Checkbox

<zk>
	<window apply="org.zkoss.bind.BindComposer"
		viewModel="@id('vm') @init('test.TestVM')">
		<listbox model="@load(vm.model)">
			<template name="model" var="each">
				<listitem>
					<listcell label="@bind(each.index)" />
					<listcell>
						<checkbox checked="@bind(each.checked)"
							onCheck="@command('updateCheckedMsg')" />
					</listcell>
				</listitem>
			</template>
		</listbox>
		<button label="add row" onClick="@command('addRow')" />
		<button label="remove checked row" onClick="@command('removeSelected')" />
		<textbox value="@load(vm.msg)" />
	</window>
</zk>

package test;

import java.util.ArrayList;
import java.util.List;

import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.ContextParam;
import org.zkoss.bind.annotation.ContextType;
import org.zkoss.bind.annotation.NotifyChange;
import org.zkoss.zk.ui.event.CheckEvent;
import org.zkoss.zul.ListModelList;
import org.zkoss.zul.Listcell;

public class TestVM {
	private ListModelList _model;
	private int _currentIndex = 0; // increase forever
	private String _msg = "";

	public ListModelList getModel () {
		if (_model == null) {
			List l = new ArrayList();
			_model = new ListModelList(l);
		}
		return _model;
	}
	public String getMsg () {
		return _msg;
	}
	@Command
	@NotifyChange({"model", "msg"})
	public void addRow () {
		_currentIndex += 1;
		_model.add(new CellCnt(_currentIndex, false));
		_msg = "Row " + _currentIndex + " is created";
	}
	@Command
	@NotifyChange({"model", "msg"})
	public void removeSelected () {
		List tmp = new ArrayList();
		for (int i = 0; i < _model.getSize(); i++) {
			if (!((CellCnt)_model.getElementAt(i)).getChecked()) {
				tmp.add(_model.getElementAt(i));
			}
		}
		_model = new ListModelList(tmp);
		_msg = "Checked rows are deleted.";
	}
	@Command
	@NotifyChange({"msg"})
	public void updateCheckedMsg (@ContextParam(ContextType.TRIGGER_EVENT) CheckEvent event) {
		String label = ((Listcell)event.getTarget().getParent().getPreviousSibling()).getLabel();
		_msg = "checkbox " + label + " is "
				+ (event.isChecked()? "checked" : "unchecked");
	}
}

package test;

public class CellCnt {
	private int _index;
	private boolean _checked;
	public CellCnt (int index, boolean checked) {
		_index = index;
		_checked = checked;
	}
	public void setIndex (int index) {
		_index = index;
	}
	public int getIndex () {
		return _index;
	}
	public void setChecked (boolean checked) {
		_checked = checked;
	}
	public boolean getChecked () {
		return _checked;
	}
}

<x:td width="38%">
						<listbox id="chosenLb" vflex="1" width="100%"
							height="135px" model="@load(vm.chosenDataModel)">
							<listhead sizable="true">
								<listheader>
									<x:table width="100%">
										<x:tr>
											<x:td width="90%">
												<label
													value="Selected Column List" style="font-weight: bold">
												</label>
											</x:td>
											<x:td width="10%">
												<label
													value="Ascending" style="font-weight: bolder">
												</label>
											</x:td>
										</x:tr>
									</x:table>
								</listheader>

							</listhead>

							<template name="model" var="selected">
								<listitem>
									<listcell>
										<x:table width="100%">
											<x:tr>
												<x:td width="90%">
													<label
														value="@bind(selected.columnHeaderName)"
														style="width:200px">
													</label>
												</x:td>
												<x:td width="10%">
													<checkbox
														checked="@bind(selected.sortAscending)"
														style="align:right;width:50px"
														onCheck="@command('checkboxClicked',item=selected.sortAscending,check=self.checked)" />
												</x:td>
											</x:tr>
										</x:table>
									</listcell>
								</listitem>
							</template>
						</listbox>
					</x:td>