GridView and AJAX PopupCalendar Issue

I tried to use ASP.NET AJAX CalendarExtender control in GridView's edit template for a date field in the way just as the example show on How Do I: Configure the ASP.NET AJAX Calendar Control? That is, I added a text box binding to a field in the gridview as date, a image button next to it as a button, and a CalendarExtender control which is AJAX control. The extender control's TargetControlID is the text box, and PopupButtonID is the image control.

However, it does not work. Actually, the pop-up calendar does pop up, but it disappears right away, and the whole page is refreshed. It looks like that the lick on image button caused a call to server to send a post-back call. The example works fine, but it is too simple(a text box, a image button and a CalendarExtender on an aspx page). I am not sure why this extender does not work when it is embedded in the gridview. I think I have to find a way to prevent the post-back call so that the pop-up calendar will stay.