OptionGroup addValueChangeListener CheckBox

package com.example.vaadindemo;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Button;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.CheckBox;
import com.vaadin.ui.Label;
import com.vaadin.ui.OptionGroup;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;

@SuppressWarnings("serial")
@Theme("vaadindemo")
public class VaadindemoUI extends UI {

    @WebServlet(value = "/*", asyncSupported = true)
    @VaadinServletConfiguration(productionMode = false, ui = VaadindemoUI.class)
    public static class Servlet extends VaadinServlet {
    }

    @Override
    protected void init(VaadinRequest request) {
        final VerticalLayout layout = new VerticalLayout();
        layout.setMargin(true);
        setContent(layout);

        OptionGroup group = new OptionGroup();
        group.addItem("01");
        group.setItemCaption("01", "ONE");

        group.addItem("02");
        group.setItemCaption("02", "TWO");

        group.addItem("03");
        group.setItemCaption("03", "THREE");

        CheckBox box = new CheckBox();
        box.setCaption("Check Me");

//Notify CheckBox of value change of radio button
        group.addValueChangeListener(box);

        layout.addComponent(group);
        layout.addComponent(box);
        
        Button button = new Button("Click Me");
        button.addClickListener(new Button.ClickListener() {
            public void buttonClick(ClickEvent event) {
                layout.addComponent(new Label("Thank you for clicking"));
            }
        });
        layout.addComponent(button);
    }

}

Vaadin 7.6.5
Why does this code fail with 

SEVERE: 
com.vaadin.data.util.converter.Converter$ConversionException: Unable to convert value of type java.lang.String to presentation type class java.lang.Boolean. No converter is set and the types are not compatible.
    at com.vaadin.data.util.converter.ConverterUtil.convertFromModel(ConverterUtil.java:116)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:736)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:721)
    at com.vaadin.ui.AbstractField.readValueFromProperty(AbstractField.java:1315)
    at com.vaadin.ui.AbstractField.valueChange(AbstractField.java:1308)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:508)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:198)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:161)
    at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:1008)
    at com.vaadin.ui.AbstractField.fireValueChange(AbstractField.java:1159)
    at com.vaadin.ui.AbstractField.setValue(AbstractField.java:570)
    at com.vaadin.ui.AbstractSelect.setValue(AbstractSelect.java:732)
    at com.vaadin.ui.AbstractField.setValue(AbstractField.java:468)
    at com.vaadin.ui.OptionGroup.setValue(OptionGroup.java:193)
    at com.vaadin.ui.AbstractSelect.changeVariables(AbstractSelect.java:541)
    at com.vaadin.ui.OptionGroup.changeVariables(OptionGroup.java:86)
    at com.vaadin.server.communication.ServerRpcHandler.changeVariables(ServerRpcHandler.java:603)
    at com.vaadin.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:422)
    at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:273)
    at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:79)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1409)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:364)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:522)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1095)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1500)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1456)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

What was the expected bevavior?