dirty solution without modifying zf/element->isValid()

application/modules/admin/controllers/OuverturesController.php

@@ -92,6 +92,9 @@ class Admin_OuverturesController extends ZendAfi_Controller_Action {
     $post = parent::_getPost();
     $post['id_site'] = $this->_getLibrary()->getId();
 
+    if(!$post['jour'])
+      $post['jour'] = 'empty';
+
     if(Class_Ouverture::AUCUN_JOUR != $post['jour_semaine'])
       $post['jour'] = null;