Exception when sendRediect from a modal dialog

It is always a exception printed in my console, int following case.

1.A main.zul which popup a modal window, which contains a redirect button.
2.When clcik the button, the page will redirect to hello.zul by Events.sendRedirect
3.In Console , You will see a execption trace :
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.LinkedList.entry(LinkedList.java:365)
at java.util.LinkedList.remove(LinkedList.java:357)
at org.zkoss.web.servlet.xel.RequestContexts.pop(RequestContexts.java:92)
at org.zkoss.zk.ui.http.ExecutionImpl.onDeactivate(ExecutionImpl.java:121)
at org.zkoss.zk.ui.impl.EventProcessingThreadImpl.run(EventProcessingThreadImpl.java:401)

>>>>main.zul
<zk>
<window title="Test">
<attribute name="onCreate">
Window window = Executions.createComponents("redirect.zul",self,null);
window.doModal();
</attribute>
</window>
</zk>

>>>>redirect.zul
<window title="Redirect" width="300px">
<button label="go home">
<attribute name="onClick">
Executions.sendRedirect("/test/hello.zul");
</attribute>
</button>
</window>
>>>>hello.zul
<zk>
welcome back
</zk>

/Dennis