Hi I get this error when trying to save a record using DateField or CalendarDateField on a CalendarDateField, any hint on debugging that stuff?

<?
class Event extends DataObject {

static $db = array(
'Lieu' => 'Text',
'CodePostal' => 'Text',
'DateDebut' => 'Date',
    'DateFin' => 'Date'
);
static $has_one = array(
'MyProject' => 'Project'
);
function getCMSFields_forPopup() {
$fields = new FieldSet();
$fields->push( new TextField( 'Lieu' ) );
$fields->push( new TextField( 'CodePostal' ) );
$fields->push( new CalendarDateField( 'DateDebut' ) );
$fields->push( new CalendarDateField( 'DateFin' ) );
return $fields;
}

}?>