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/lang/en/attendance.php b/lang/en/attendance.php
index 68f5477..60788fd 100644
--- a/lang/en/attendance.php
+++ b/lang/en/attendance.php
@@ -126,6 +126,7 @@ $string['includenottaken'] = 'Include not taken sessions';
 $string['includeremarks'] = 'Include remarks';
 $string['indetail'] = 'In detail...';
 $string['invalidsessionenddate'] = 'The session end date can not be earlier than the session start date';
+$string['invalidaction'] = 'You must select an action';
 $string['jumpto'] = 'Jump to';
 $string['modulename'] = 'Attendance';
 $string['modulename_help'] = 'The attendance activity module enables a teacher to take attendance during class and students to view their own attendance record.
diff --git a/renderer.php b/renderer.php
index 7138e15..d8d17c1 100644
--- a/renderer.php
+++ b/renderer.php
@@ -294,6 +294,7 @@ class mod_attendance_renderer extends plugin_renderer_base {
             $options = array(
                         att_sessions_page_params::ACTION_DELETE_SELECTED => get_string('delete'),
                         att_sessions_page_params::ACTION_CHANGE_DURATION => get_string('changeduration', 'attendance'));
+
             $controls = html_writer::select($options, 'action');
             $attributes = array(
                     'type'  => 'submit',
diff --git a/sessions.php b/sessions.php
index fda84fe..1bc08f5 100644
--- a/sessions.php
+++ b/sessions.php
@@ -33,6 +33,13 @@ $pageparams = new att_sessions_page_params();
 $id                     = required_param('id', PARAM_INT);
 $pageparams->action     = required_param('action', PARAM_INT);
 
+if (empty($pageparams->action)) {
+    // The form on manage.php can submit with the "choose" option - this should be fixed in the long term,
+    // but in the meantime show a useful error and redirect when it occurs.
+    $url = new moodle_url('/mod/attendance/view.php', array('id' => $id));
+    redirect($url, get_string('invalidaction', 'mod_attendance'), 2);
+}
+
 $cm             = get_coursemodule_from_id('attendance', $id, 0, false, MUST_EXIST);
 $course         = $DB->get_record('course', array('id' => $cm->course), '*', MUST_EXIST);
 $att            = $DB->get_record('attendance', array('id' => $cm->instance), '*', MUST_EXIST);
@@ -119,7 +126,7 @@ switch ($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);
 
@@ -142,10 +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;