diff --git a/course/externallib.php b/course/externallib.php
index f38953a354c..ddede57b623 100644
--- a/course/externallib.php
+++ b/course/externallib.php
@@ -756,6 +756,26 @@ class core_course_external extends external_api {
         );
     }
 
+    /**
+     * Return array of all editable course custom fields indexed by their shortname
+     *
+     * @param \context $context
+     * @param int $courseid
+     * @return \core_customfield\field_controller[]
+     */
+    public static function get_editable_customfields(\context $context, int $courseid = 0): array {
+        $result = [];
+
+        $handler = \core_course\customfield\course_handler::create();
+        $handler->set_parent_context($context);
+
+        foreach ($handler->get_editable_fields($courseid) as $field) {
+            $result[$field->get('shortname')] = $field;
+        }
+
+        return $result;
+    }
+
     /**
      * Returns description of method parameters
      *
@@ -928,8 +948,13 @@ class core_course_external extends external_api {
 
             // Custom fields.
             if (!empty($course['customfields'])) {
+                $customfields = self::get_editable_customfields($context);
                 foreach ($course['customfields'] as $field) {
-                    $course['customfield_'.$field['shortname']] = $field['value'];
+                    if (array_key_exists($field['shortname'], $customfields)) {
+                        // Ensure we're populating the element form fields correctly.
+                        $controller = \core_customfield\data_controller::create(0, null, $customfields[$field['shortname']]);
+                        $course[$controller->get_form_element_name()] = $field['value'];
+                    }
                 }
             }
 
@@ -1141,10 +1166,15 @@ class core_course_external extends external_api {
                     }
                 }
 
-                // Prepare list of custom fields.
+                // Custom fields.
                 if (isset($course['customfields'])) {
+                    $customfields = self::get_editable_customfields($context, $course['id']);
                     foreach ($course['customfields'] as $field) {
-                        $course['customfield_' . $field['shortname']] = $field['value'];
+                        if (array_key_exists($field['shortname'], $customfields)) {
+                            // Ensure we're populating the element form fields correctly.
+                            $controller = \core_customfield\data_controller::create(0, null, $customfields[$field['shortname']]);
+                            $course[$controller->get_form_element_name()] = $field['value'];
+                        }
                     }
                 }
 
diff --git a/course/tests/externallib_test.php b/course/tests/externallib_test.php
index 3911a36d977..e71b974c543 100644
--- a/course/tests/externallib_test.php
+++ b/course/tests/externallib_test.php
@@ -453,10 +453,12 @@ class externallib_test extends externallib_advanced_testcase {
         // Custom fields.
         $fieldcategory = self::getDataGenerator()->create_custom_field_category(['name' => 'Other fields']);
 
-        $customfield = ['shortname' => 'test', 'name' => 'Custom field', 'type' => 'text',
-            'categoryid' => $fieldcategory->get('id'),
-            'configdata' => ['visibility' => \core_course\customfield\course_handler::VISIBLETOALL]];
-        $field = self::getDataGenerator()->create_custom_field($customfield);
+        $fieldtext = self::getDataGenerator()->create_custom_field([
+            'categoryid' => $fieldcategory->get('id'), 'name' => 'Text', 'shortname' => 'text', 'type' => 'text',
+        ]);
+        $fieldtextarea = self::getDataGenerator()->create_custom_field([
+            'categoryid' => $fieldcategory->get('id'), 'name' => 'Textarea', 'shortname' => 'textarea', 'type' => 'textarea',
+        ]);
 
         // Set the required capabilities by the external function
         $contextid = context_system::instance()->id;
@@ -508,7 +510,10 @@ class externallib_test extends externallib_advanced_testcase {
         $course4['fullname'] = 'Test course with custom fields';
         $course4['shortname'] = 'Testcoursecustomfields';
         $course4['categoryid'] = $category->id;
-        $course4['customfields'] = [['shortname' => $customfield['shortname'], 'value' => 'Test value']];
+        $course4['customfields'] = [
+            ['shortname' => $fieldtext->get('shortname'), 'value' => 'And I want to tell you so much'],
+            ['shortname' => $fieldtextarea->get('shortname'), 'value' => 'I love you'],
+        ];
         $courses = array($course4, $course1, $course2, $course3);
 
         $createdcourses = core_course_external::create_courses($courses);
@@ -580,7 +585,10 @@ class externallib_test extends externallib_advanced_testcase {
 
                 $handler = core_course\customfield\course_handler::create();
                 $customfields = $handler->export_instance_data_object($createdcourse['id']);
-                $this->assertEquals((object)['test' => 'Test value'], $customfields);
+                $this->assertEquals((object) [
+                    'text' => 'And I want to tell you so much',
+                    'textarea' => '<div class="text_to_html">I love you</div>',
+                ], $customfields);
             } else {
                 throw new moodle_exception('Unexpected shortname');
             }
@@ -1857,12 +1865,17 @@ class externallib_test extends externallib_advanced_testcase {
 
         // Course with custom fields.
         $fieldcategory = self::getDataGenerator()->create_custom_field_category(['name' => 'Other fields']);
-        $customfield = ['shortname' => 'test', 'name' => 'Custom field', 'type' => 'text',
-            'categoryid' => $fieldcategory->get('id'),
-            'configdata' => ['visibility' => \core_course\customfield\course_handler::VISIBLETOALL, 'locked' => 1]];
-        $field = self::getDataGenerator()->create_custom_field($customfield);
 
-        $originalcourse3 = self::getDataGenerator()->create_course(['customfield_test' => 'Test value']);
+        $fieldtext = self::getDataGenerator()->create_custom_field([
+            'categoryid' => $fieldcategory->get('id'), 'name' => 'Text', 'shortname' => 'text', 'type' => 'text', 'configdata' => [
+                'locked' => 1,
+            ],
+        ]);
+        $fieldtextarea = self::getDataGenerator()->create_custom_field([
+            'categoryid' => $fieldcategory->get('id'), 'name' => 'Textarea', 'shortname' => 'textarea', 'type' => 'textarea',
+        ]);
+
+        $originalcourse3 = self::getDataGenerator()->create_course();
         self::getDataGenerator()->enrol_user($USER->id, $originalcourse3->id, $roleid);
 
         // Course values to be updated.
@@ -1895,8 +1908,11 @@ class externallib_test extends externallib_advanced_testcase {
         $course2['forcetheme'] = 'classic';
 
         $course3['id'] = $originalcourse3->id;
-        $updatedcustomfieldvalue = ['shortname' => 'test', 'value' => 'Updated test value'];
-        $course3['customfields'] = [$updatedcustomfieldvalue];
+        $course3['customfields'] = [
+            ['shortname' => $fieldtext->get('shortname'), 'value' => 'I long to see the sunlight in your hair'],
+            ['shortname' => $fieldtextarea->get('shortname'), 'value' => 'And tell you time and time again'],
+         ];
+
         $courses = array($course1, $course2, $course3);
 
         $updatedcoursewarnings = core_course_external::update_courses($courses);
@@ -1937,7 +1953,10 @@ class externallib_test extends externallib_advanced_testcase {
                 }
 
                 $this->assertEquals($course2['enablecompletion'], $courseinfo->enablecompletion);
-                $this->assertEquals(['test' => null], (array)$customfields);
+                $this->assertEquals((object) [
+                    'text' => null,
+                    'textarea' => null,
+                ], $customfields);
             } else if ($course['id'] == $course1['id']) {
                 $this->assertEquals($course1['fullname'], $courseinfo->fullname);
                 $this->assertEquals($course1['shortname'], $courseinfo->shortname);
@@ -1947,9 +1966,15 @@ class externallib_test extends externallib_advanced_testcase {
                 $this->assertEquals(5, course_get_format($course['id'])->get_last_section_number());
                 $this->assertEquals(0, $courseinfo->newsitems);
                 $this->assertEquals(FORMAT_MOODLE, $courseinfo->summaryformat);
-                $this->assertEquals(['test' => null], (array)$customfields);
+                $this->assertEquals((object) [
+                    'text' => null,
+                    'textarea' => null,
+                ], $customfields);
             } else if ($course['id'] == $course3['id']) {
-                $this->assertEquals(['test' => $updatedcustomfieldvalue['value']], (array)$customfields);
+                $this->assertEquals((object) [
+                    'text' => 'I long to see the sunlight in your hair',
+                    'textarea' => '<div class="text_to_html">And tell you time and time again</div>',
+                ], $customfields);
             } else {
                 throw new moodle_exception('Unexpected shortname');
             }
@@ -2087,14 +2112,18 @@ class externallib_test extends externallib_advanced_testcase {
         $this->setUser($user);
         self::getDataGenerator()->enrol_user($user->id, $course3['id'], $roleid);
 
-        $newupdatedcustomfieldvalue = ['shortname' => 'test', 'value' => 'New updated value'];
-        $course3['customfields'] = [$newupdatedcustomfieldvalue];
+        $course3['customfields'] = [
+            ['shortname' => 'text', 'value' => 'New updated value'],
+        ];
 
         core_course_external::update_courses([$course3]);
 
         // Custom field was not updated.
         $customfields = \core_course\customfield\course_handler::create()->export_instance_data_object($course3['id']);
-        $this->assertEquals(['test' => $updatedcustomfieldvalue['value']], (array)$customfields);
+        $this->assertEquals((object) [
+            'text' => 'I long to see the sunlight in your hair',
+            'textarea' => '<div class="text_to_html">And tell you time and time again</div>',
+        ], $customfields);
     }
 
     /**
diff --git a/customfield/field/textarea/classes/data_controller.php b/customfield/field/textarea/classes/data_controller.php
index 3d52fe15bc4..a72d99c8bd3 100644
--- a/customfield/field/textarea/classes/data_controller.php
+++ b/customfield/field/textarea/classes/data_controller.php
@@ -88,7 +88,14 @@ class data_controller extends \core_customfield\data_controller {
         if (!property_exists($datanew, $fieldname)) {
             return;
         }
+
+        // Normalise form data, for cases it's come from an external source.
         $fromform = $datanew->$fieldname;
+        if (!is_array($fromform)) {
+            $fromform = ['text' => $fromform];
+            $fromform['format'] = $this->get('id') ? $this->get('valueformat') :
+                $this->get_field()->get_configdata_property('defaultvalueformat');
+        }
 
         if (!$this->get('id')) {
             $this->data->set('value', '');
diff --git a/customfield/tests/generator/lib.php b/customfield/tests/generator/lib.php
index 06cfe479f97..93a85c6a3d2 100644
--- a/customfield/tests/generator/lib.php
+++ b/customfield/tests/generator/lib.php
@@ -124,6 +124,7 @@ class core_customfield_generator extends component_generator_base {
             'locked' => 0,
             'visibility' => 2,
             'defaultvalue' => '',
+            'defaultvalueformat' => FORMAT_MOODLE,
             'displaysize' => 0,
             'maxlength' => 0,
             'ispassword' => 0,