diff --git a/duration_form.php b/duration_form.php
index 92877ba..3051642 100644
--- a/duration_form.php
+++ b/duration_form.php
@@ -61,8 +61,11 @@ class mod_attendance_duration_form extends moodleform {
         $mform->addGroup($durselect, 'durtime', get_string('newduration', 'attendance'), array(' '), true);
 
         $mform->addElement('hidden', 'ids', $ids);
+        $mform->setType('ids', PARAM_ALPHANUMEXT);
         $mform->addElement('hidden', 'id', $cm->id);
+        $mform->setType('id', PARAM_INT);
         $mform->addElement('hidden', 'action', att_sessions_page_params::ACTION_CHANGE_DURATION);
+        $mform->setType('action', PARAM_INT);
 
         $mform->setDefaults(array('durtime' => array('hours'=>0, 'minutes'=>0)));
 
diff --git a/sessions.php b/sessions.php
index 2b60e57..1bc08f5 100644
--- a/sessions.php
+++ b/sessions.php
@@ -126,7 +126,7 @@ switch ($att->pageparams->action) {
             }
             redirect($att->url_manage(), get_string('sessiondeleted', 'attendance'));
         }
-        $sessid = required_param('sessid', PARAM_SEQUENCE);
+        $sessid = required_param_array('sessid', PARAM_SEQUENCE);
 
         $sessionsinfo = $att->get_sessions_info($sessid);
 
@@ -149,10 +149,10 @@ switch ($att->pageparams->action) {
         echo $OUTPUT->footer();
         exit;
     case att_sessions_page_params::ACTION_CHANGE_DURATION:
-        $sessid = optional_param('sessid', '', PARAM_SEQUENCE);
+        $sessid = optional_param_array('sessid', '', PARAM_SEQUENCE);
         $ids = optional_param('ids', '', PARAM_ALPHANUMEXT);
 
-        $slist = isset($sessid) ? implode('_', $sessid) : '';
+        $slist = !empty($sessid) ? implode('_', $sessid) : '';
 
         $url = $att->url_sessions(array('action' => att_sessions_page_params::ACTION_CHANGE_DURATION));
         $formparams['ids'] = $slist;