diff --git a/lib/formslib.php b/lib/formslib.php
index 6dba9626520..70778e56e9f 100644
--- a/lib/formslib.php
+++ b/lib/formslib.php
@@ -2250,11 +2250,13 @@ class MoodleQuickForm extends HTML_QuickForm_DHTMLRulesTableless {
             // iterate over all elements, calling their exportValue() methods
             foreach (array_keys($this->_elements) as $key) {
                 if ($this->_elements[$key]->isFrozen() && !$this->_elements[$key]->_persistantFreeze) {
-                    $varname = $this->_elements[$key]->_attributes['name'];
                     $value = '';
-                    // If we have a default value then export it.
-                    if (isset($this->_defaultValues[$varname])) {
-                        $value = $this->prepare_fixed_value($varname, $this->_defaultValues[$varname]);
+                    if (isset($this->_elements[$key]->_attributes['name'])) {
+                        $varname = $this->_elements[$key]->_attributes['name'];
+                        // If we have a default value then export it.
+                        if (isset($this->_defaultValues[$varname])) {
+                            $value = $this->prepare_fixed_value($varname, $this->_defaultValues[$varname]);
+                        }
                     }
                 } else {
                     $value = $this->_elements[$key]->exportValue($this->_submitValues, true);
diff --git a/mod/quiz/lib.php b/mod/quiz/lib.php
index c0209589272..3b0a042c2c7 100644
--- a/mod/quiz/lib.php
+++ b/mod/quiz/lib.php
@@ -1128,14 +1128,17 @@ function quiz_process_options($quiz) {
     $quiz->reviewoverallfeedback &= ~mod_quiz_display_options::DURING;
 
     // Ensure that disabled checkboxes in completion settings are set to 0.
-    if (empty($quiz->completionusegrade)) {
-        $quiz->completionpass = 0;
-    }
-    if (empty($quiz->completionpass)) {
-        $quiz->completionattemptsexhausted = 0;
-    }
-    if (empty($quiz->completionminattemptsenabled)) {
-        $quiz->completionminattempts = 0;
+    // But only if the completion settinsg are unlocked.
+    if (!empty($quiz->completionunlocked)) {
+        if (empty($quiz->completionusegrade)) {
+            $quiz->completionpass = 0;
+        }
+        if (empty($quiz->completionpass)) {
+            $quiz->completionattemptsexhausted = 0;
+        }
+        if (empty($quiz->completionminattemptsenabled)) {
+            $quiz->completionminattempts = 0;
+        }
     }
 }
 
diff --git a/mod/quiz/tests/behat/quiz_activity_completion_locked.feature b/mod/quiz/tests/behat/quiz_activity_completion_locked.feature
new file mode 100644
index 00000000000..5bdc69069ff
--- /dev/null
+++ b/mod/quiz/tests/behat/quiz_activity_completion_locked.feature
@@ -0,0 +1,56 @@
+@mod @mod_quiz @core_completion
+Feature: Ensure saving a quiz does not modify the completion settings.
+  In order to reliably use completion
+  As a teacher
+  I need to be able to update the quiz
+  without changing the completion settings.
+
+  Background:
+    Given the following "users" exist:
+      | username | firstname | lastname | email                |
+      | student1 | Student   | 1        | student1@example.com |
+      | teacher1 | Teacher   | 1        | teacher1@example.com |
+    And the following "courses" exist:
+      | fullname | shortname | category | enablecompletion |
+      | Course 1 | C1        | 0        | 1                |
+    And the following "course enrolments" exist:
+      | user     | course | role           |
+      | teacher1 | C1     | editingteacher |
+      | student1 | C1     | student        |
+    And the following config values are set as admin:
+      | grade_item_advanced | hiddenuntil |
+    And the following "question categories" exist:
+      | contextlevel | reference | name           |
+      | Course       | C1        | Test questions |
+    And the following "questions" exist:
+      | questioncategory | qtype     | name           | questiontext              |
+      | Test questions   | truefalse | First question | Answer the first question |
+    And the following "activity" exists:
+      | activity                     | quiz      |
+      | course                       | C1        |
+      | idnumber                     | quiz1     |
+      | name                         | Test quiz |
+      | section                      | 1         |
+      | attempts                     | 2         |
+      | gradepass                    | 5.00      |
+      | completion                   | 2         |
+      | completionview               | 0         |
+      | completionusegrade           | 1         |
+      | completionpass               | 1         |
+      | completionattemptsexhausted  | 1         |
+    And quiz "Test quiz" contains the following questions:
+      | question       | page |
+      | First question | 1    |
+    And user "student1" has attempted "Test quiz" with responses:
+      | slot | response |
+      |   1  | True     |
+
+  Scenario: Ensure saving quiz activty does not change completion settings
+    Given I am on the "Test quiz" "mod_quiz > View" page logged in as "teacher1"
+    When I navigate to "Edit settings" in current page administration
+    Then the "completionattemptsexhausted" "field" should be disabled
+    And the field "completionattemptsexhausted" matches value "1"
+    And I press "Save and display"
+    And I navigate to "Edit settings" in current page administration
+    And the "completionattemptsexhausted" "field" should be disabled
+    And the field "completionattemptsexhausted" matches value "1"