diff --git a/lib/classes/external/exporter.php b/lib/classes/external/exporter.php
index bfaa6ef0918..71cd67e9277 100644
--- a/lib/classes/external/exporter.php
+++ b/lib/classes/external/exporter.php
@@ -425,10 +425,12 @@ abstract class exporter {
      * @return external_format_value
      */
     final protected static function get_format_structure($property, $definition, $required = VALUE_REQUIRED) {
+        $default = null;
         if (array_key_exists('default', $definition)) {
             $required = VALUE_DEFAULT;
+            $default = $definition['default'];
         }
-        return new external_format_value($property, $required);
+        return new external_format_value($property, $required, $default);
     }
 
     /**
@@ -547,7 +549,12 @@ abstract class exporter {
                     if (isset($returns[$formatproperty])) {
                         throw new coding_exception('The format for \'' . $property . '\' is already defined.');
                     }
-                    $returns[$formatproperty] = self::get_format_structure($property, $properties[$formatproperty]);
+                    $formatpropertydef = $properties[$formatproperty];
+                    $formatpropertyrequired = VALUE_REQUIRED;
+                    if (!empty($formatpropertydef['optional'])) {
+                        $formatpropertyrequired = VALUE_OPTIONAL;
+                    }
+                    $returns[$formatproperty] = self::get_format_structure($property, $formatpropertydef, $formatpropertyrequired);
                 }
             }
         }
diff --git a/lib/external/classes/external_format_value.php b/lib/external/classes/external_format_value.php
index e13c68e1120..71610aa0520 100644
--- a/lib/external/classes/external_format_value.php
+++ b/lib/external/classes/external_format_value.php
@@ -38,12 +38,19 @@ class external_format_value extends external_value {
      * @param int $default Default value.
      */
     public function __construct($textfieldname, $required = VALUE_REQUIRED, $default = null) {
+        // Make sure the default format's value is correct.
+        if ($default !== null && !in_array($default, [FORMAT_MOODLE, FORMAT_HTML, FORMAT_PLAIN, FORMAT_MARKDOWN])) {
+            debugging("Invalid default format for $textfieldname: $default. " .
+                      "It must be either FORMAT_MOODLE, FORMAT_HTML, FORMAT_PLAIN, or FORMAT_MARKDOWN.", DEBUG_DEVELOPER);
+            $default = null;
+        }
+
         if ($default == null && $required == VALUE_DEFAULT) {
             $default = FORMAT_HTML;
         }
 
         $desc = sprintf(
-            "%s format (%s = HTML, %s = MOODLE, %s = PLAIN, or %s = MARKDOWN",
+            "%s format (%s = HTML, %s = MOODLE, %s = PLAIN, or %s = MARKDOWN)",
             $textfieldname,
             FORMAT_HTML,
             FORMAT_MOODLE,
diff --git a/lib/tests/exporter_test.php b/lib/tests/exporter_test.php
index edb1e5ae93f..baed4f8de69 100644
--- a/lib/tests/exporter_test.php
+++ b/lib/tests/exporter_test.php
@@ -203,24 +203,98 @@ class exporter_test extends \advanced_testcase {
         $this->assertEquals('id', $properties['nestedarray']['type']['id']['description']);
     }
 
-    public function test_format_properties_with_optional() {
+    /**
+     * Test the processing of format properties.
+     *
+     * @covers \core\external\exporter::get_read_structure
+     * @return void
+     */
+    public function test_format_properties_with_optional(): void {
         $testable = new class([]) extends \core\external\exporter {
-        public static function define_properties() {
-            return [
-                'content' => [
-                    'type' => PARAM_RAW,
-                    'optional' => true,
-                ],
-                'contentformat' => [
-                    'type' => PARAM_INT,
-                    'optional' => true,
-                ]
-            ];
-        }
+            /**
+             * Properties definition.
+             *
+             * @return array[]
+             */
+            public static function define_properties(): array {
+                return [
+                    'content' => [
+                        'type' => PARAM_RAW,
+                    ],
+                    'contentformat' => [
+                        'type' => PARAM_INT,
+                        'optional' => true,
+                    ],
+                    'description' => [
+                        'type' => PARAM_RAW,
+                        'optional' => true,
+                    ],
+                    'descriptionformat' => [
+                        'type' => PARAM_INT,
+                        'default' => FORMAT_MARKDOWN,
+                    ],
+                    'summary' => [
+                        'type' => PARAM_RAW,
+                    ],
+                    'summaryformat' => [
+                        'type' => PARAM_INT,
+                        'default' => null,
+                    ],
+                ];
+            }
         };
+
         $definition = $testable::get_read_structure();
-        $this->assertEquals(VALUE_OPTIONAL, $definition->keys['content']->required);
+        // Check content and its format.
+        $this->assertEquals(VALUE_REQUIRED, $definition->keys['content']->required);
         $this->assertEquals(VALUE_OPTIONAL, $definition->keys['contentformat']->required);
+        $this->assertEquals(null, $definition->keys['contentformat']->default);
+
+        // Check description and its format.
+        $this->assertEquals(VALUE_OPTIONAL, $definition->keys['description']->required);
+        $this->assertEquals(VALUE_DEFAULT, $definition->keys['descriptionformat']->required);
+        $this->assertEquals(FORMAT_MARKDOWN, $definition->keys['descriptionformat']->default);
+
+        // Check summary and its format.
+        $this->assertEquals(VALUE_REQUIRED, $definition->keys['summary']->required);
+        $this->assertEquals(null, $definition->keys['summary']->default);
+        $this->assertEquals(VALUE_DEFAULT, $definition->keys['summaryformat']->required);
+        $this->assertEquals(FORMAT_HTML, $definition->keys['summaryformat']->default);
+    }
+
+    /**
+     * Test the processing of format properties when an invalid default format is passed.
+     *
+     * @covers \core\external\exporter::get_read_structure
+     * @return void
+     */
+    public function test_optional_format_property_with_invalid_default(): void {
+        $testable = new class([]) extends \core\external\exporter {
+            /**
+             * Properties definition.
+             *
+             * @return array[]
+             */
+            public static function define_properties(): array {
+                return [
+                    'description' => [
+                        'type' => PARAM_RAW,
+                    ],
+                    'descriptionformat' => [
+                        'type' => PARAM_INT,
+                        'default' => 999,
+                    ],
+                ];
+            }
+        };
+
+        $definition = $testable::get_read_structure();
+        $this->assertDebuggingCalled(null, DEBUG_DEVELOPER);
+
+        // Check description and its format.
+        $this->assertEquals(VALUE_REQUIRED, $definition->keys['description']->required);
+        $this->assertEquals(VALUE_DEFAULT, $definition->keys['descriptionformat']->required);
+        $this->assertEquals(FORMAT_HTML, $definition->keys['descriptionformat']->default);
     }
 }