AjaxCheckBox not receiving onUpdate Event

[Advanced Technology®]

Hi,
I have this code :
            final AjaxCheckBox bbcaucao = new AjaxCheckBox("bbcaucao") {
                protected void onUpdate(AjaxRequestTarget target) {
                    System.out.println(new Date());
                }
            };

The Date gets printed only if the checkbox is  unchecked.

Could someone give me some light.

Thanks in Advance.
AT(R)