diff --git a/cohort/edit_form.php b/cohort/edit_form.php
index fe3f35754f0..249212c7ae1 100644
--- a/cohort/edit_form.php
+++ b/cohort/edit_form.php
@@ -49,7 +49,7 @@ class cohort_edit_form extends moodleform {
         $mform->addElement('select', 'contextid', get_string('context', 'role'), $options);
 
         $mform->addElement('text', 'idnumber', get_string('idnumber', 'cohort'), 'maxlength="254" size="50"');
-        $mform->setType('name', PARAM_RAW);
+        $mform->setType('name', PARAM_RAW); // idnumbers are plain text, must not be changed
 
         $mform->addElement('editor', 'description_editor', get_string('description', 'cohort'), null, $editoroptions);
         $mform->setType('description_editor', PARAM_RAW);
diff --git a/cohort/index.php b/cohort/index.php
index ba656e0ce72..fd8c9cbefe8 100644
--- a/cohort/index.php
+++ b/cohort/index.php
@@ -75,7 +75,7 @@ $data = array();
 foreach($cohorts as $cohort) {
     $line = array();
     $line[] = format_string($cohort->name);
-    $line[] = $cohort->idnumber;
+    $line[] = s($cohort->idnumber); // plain text
     $line[] = format_text($cohort->description, $cohort->descriptionformat);
 
     $line[] = $DB->count_records('cohort_members', array('cohortid'=>$cohort->id));