diff --git a/mod/lesson/mod_form.php b/mod/lesson/mod_form.php
index 859666b25af..33774899955 100644
--- a/mod/lesson/mod_form.php
+++ b/mod/lesson/mod_form.php
@@ -337,6 +337,13 @@ class mod_lesson_mod_form extends moodleform_mod {
         if (empty($data['maxtime']) and !empty($data['timed'])) {
             $errors['timedgrp'] = get_string('err_numeric', 'form');
         }
+
+        // Check open and close times are consistent.
+        if ($data['available'] != 0 && $data['deadline'] != 0 &&
+                $data['deadline'] < $data['available']) {
+            $errors['deadline'] = get_string('closebeforeopen', 'lesson');
+        }
+
         if (!empty($data['usepassword']) && empty($data['password'])) {
             $errors['password'] = get_string('emptypassword', 'lesson');
         }