diff --git a/mod/assign/override_form.php b/mod/assign/override_form.php
index 33f2e0ee8d0..ab732b5e74d 100644
--- a/mod/assign/override_form.php
+++ b/mod/assign/override_form.php
@@ -40,7 +40,7 @@ class assign_override_form extends moodleform {
     /** @var object course module object. */
     protected $cm;
 
-    /** @var object the assign settings object. */
+    /** @var assign the assign settings object. */
     protected $assign;
 
     /** @var context the assign context. */
@@ -229,8 +229,10 @@ class assign_override_form extends moodleform {
             }
         }
 
+        // Determine users from which we will calculate maximum extension due date (those from current group plus selected user).
         $users = $DB->get_fieldset_select('groups_members', 'userid', 'groupid = ?', array($this->groupid));
-        array_push($users, $this->userid);
+        array_push($users, $userid);
+
         $extensionmax = 0;
         foreach ($users as $value) {
             $extension = $DB->get_record('assign_user_flags', array('assignment' => $assigninstance->id,
diff --git a/mod/assign/tests/behat/assign_user_override.feature b/mod/assign/tests/behat/assign_user_override.feature
index 1602cad4bb3..b07c85eeb78 100644
--- a/mod/assign/tests/behat/assign_user_override.feature
+++ b/mod/assign/tests/behat/assign_user_override.feature
@@ -82,6 +82,30 @@ Feature: Assign user override
     And I am on the "Test assignment name" Activity page logged in as student1
     And the activity date in "Test assignment name" should contain "Due: Wednesday, 1 January 2020, 8:00"
 
+  @javascript
+  Scenario: Ensure an overridden due date is before any extension date
+    Given I am on the "Test assignment name" Activity page logged in as teacher1
+    When I navigate to "Settings" in current page administration
+    And I set the field "Due date" to "##1 Jan 2000 08:00##"
+    And I press "Save and display"
+    And I navigate to "Submissions" in current page administration
+    And I open the action menu in "Student1" "table_row"
+    And I follow "Grant extension"
+    And I set the field "Extension due date" to "##3 Jan 2000 08:00##"
+    And I press "Save changes"
+    And I navigate to "Overrides" in current page administration
+    And I press "Add user override"
+    And I set the following fields to these values:
+      | Override user | Student1             |
+      | Due date      | ##4 Jan 2000 08:00## |
+    And I press "Save"
+    Then I should see "Extension date must be after the due date"
+    And I set the field "Due date" to "##2 Jan 2000 08:00##"
+    And I press "Save"
+    And the following should exist in the "generaltable" table:
+      | User          | Overrides | -3-                          |
+      | Sam1 Student1 | Due date  | Sunday, 2 January 2000, 8:00 |
+
   @javascript
   Scenario: Allow a user to have a different cut off date
     Given I am on the "Test assignment name" Activity page logged in as teacher1