diff --git a/calendar/set.php b/calendar/set.php
index 345f9e7d3ff..fab8aa54c02 100644
--- a/calendar/set.php
+++ b/calendar/set.php
@@ -41,8 +41,6 @@
 require_once('../config.php');
 require_once($CFG->dirroot.'/calendar/lib.php');
 
-require_sesskey();
-
 $var = required_param('var', PARAM_ALPHA);
 $return = clean_param(base64_decode(required_param('return', PARAM_RAW)), PARAM_LOCALURL);
 $courseid = optional_param('id', -1, PARAM_INT);
@@ -51,6 +49,12 @@ if ($courseid != -1) {
 } else {
     $return = new moodle_url($return);
 }
+
+if (!confirm_sesskey()) {
+    // Do not call require_sesskey() since this page may be accessed without session (for example by bots).
+    redirect($return);
+}
+
 $url = new moodle_url('/calendar/set.php', array('return'=>base64_encode($return->out_as_local_url(false)), 'course' => $courseid, 'var'=>$var, 'sesskey'=>sesskey()));
 $PAGE->set_url($url);
 $PAGE->set_context(context_system::instance());