diff --git a/lib/external/classes/external_api.php b/lib/external/classes/external_api.php
index 5bd51bd93c7aa..c770a80ab7158 100644
--- a/lib/external/classes/external_api.php
+++ b/lib/external/classes/external_api.php
@@ -312,6 +312,9 @@ public static function set_timeout($seconds = 360) {
      * @since Moodle 2.0
      */
     public static function validate_parameters(external_description $description, $params) {
+        if ($params === null && $description->allownull == NULL_ALLOWED) {
+            return null;
+        }
         if ($description instanceof external_value) {
             if (is_array($params) || is_object($params)) {
                 throw new invalid_parameter_exception('Scalar type expected, array or object received.');
@@ -398,6 +401,9 @@ public static function validate_parameters(external_description $description, $p
      * @since Moodle 2.0
      */
     public static function clean_returnvalue(external_description $description, $response) {
+        if ($response === null && $description->allownull == NULL_ALLOWED) {
+            return null;
+        }
         if ($description instanceof external_value) {
             if (is_array($response) || is_object($response)) {
                 throw new invalid_response_exception('Scalar type expected, array or object received.');
diff --git a/lib/external/classes/external_description.php b/lib/external/classes/external_description.php
index ef44a87acf796..6deea883fb549 100644
--- a/lib/external/classes/external_description.php
+++ b/lib/external/classes/external_description.php
@@ -33,14 +33,18 @@ abstract class external_description {
     /** @var mixed Default value */
     public $default;
 
+    /** @var bool Allow null values */
+    public $allownull;
+
     /**
      * Contructor.
      *
      * @param string $desc Description of element
      * @param int $required Whether the element value is required. Valid values are VALUE_DEFAULT, VALUE_REQUIRED, VALUE_OPTIONAL.
      * @param mixed $default The default value
+     * @param bool $allownull Allow null value
      */
-    public function __construct($desc, $required, $default) {
+    public function __construct($desc, $required, $default, $allownull = NULL_NOTALLOWED) {
         if (!in_array($required, [VALUE_DEFAULT, VALUE_REQUIRED, VALUE_OPTIONAL], true)) {
             $requiredstr = $required;
             if (is_array($required)) {
@@ -52,5 +56,6 @@ public function __construct($desc, $required, $default) {
         $this->desc = $desc;
         $this->required = $required;
         $this->default = $default;
+        $this->allownull = (bool)$allownull;
     }
 }
diff --git a/lib/external/classes/external_function_parameters.php b/lib/external/classes/external_function_parameters.php
index 73ec95d969f3a..085e91d3a8d04 100644
--- a/lib/external/classes/external_function_parameters.php
+++ b/lib/external/classes/external_function_parameters.php
@@ -50,6 +50,6 @@ public function __construct(
                 }
             }
         }
-        parent::__construct($keys, $desc, $required, $default);
+        parent::__construct($keys, $desc, $required, $default, NULL_NOT_ALLOWED);
     }
 }
diff --git a/lib/external/classes/external_multiple_structure.php b/lib/external/classes/external_multiple_structure.php
index 3a16c8a32b090..8362b818e0a95 100644
--- a/lib/external/classes/external_multiple_structure.php
+++ b/lib/external/classes/external_multiple_structure.php
@@ -35,14 +35,16 @@ class external_multiple_structure extends external_description {
      * @param string $desc
      * @param int $required
      * @param array $default
+     * @param bool $allownull
      */
     public function __construct(
         external_description $content,
         $desc = '',
         $required = VALUE_REQUIRED,
-        $default = null
+        $default = null,
+        $allownull = NULL_NOT_ALLOWED
     ) {
-        parent::__construct($desc, $required, $default);
+        parent::__construct($desc, $required, $default, $allownull);
         $this->content = $content;
     }
 }
diff --git a/lib/external/classes/external_single_structure.php b/lib/external/classes/external_single_structure.php
index 52231aaff4d6a..7aa7bad0a89de 100644
--- a/lib/external/classes/external_single_structure.php
+++ b/lib/external/classes/external_single_structure.php
@@ -35,14 +35,16 @@ class external_single_structure extends external_description {
      * @param string $desc
      * @param int $required
      * @param array $default
+     * @param bool $allownull
      */
     public function __construct(
         array $keys,
         $desc = '',
         $required = VALUE_REQUIRED,
-        $default = null
+        $default = null,
+        $allownull = NULL_NOT_ALLOWED
     ) {
-        parent::__construct($desc, $required, $default);
+        parent::__construct($desc, $required, $default, $allownull);
         $this->keys = $keys;
     }
 }
diff --git a/lib/external/classes/external_value.php b/lib/external/classes/external_value.php
index 45aef862b035b..037441e29dd98 100644
--- a/lib/external/classes/external_value.php
+++ b/lib/external/classes/external_value.php
@@ -28,9 +28,6 @@ class external_value extends external_description {
     /** @var mixed Value type PARAM_XX */
     public $type;
 
-    /** @var bool Allow null values */
-    public $allownull;
-
     /**
      * Constructor for the external_value class.
      *
@@ -47,8 +44,7 @@ public function __construct(
         $default = null,
         $allownull = NULL_ALLOWED
     ) {
-        parent::__construct($desc, $required, $default);
+        parent::__construct($desc, $required, $default, $allownull);
         $this->type = $type;
-        $this->allownull = $allownull;
     }
 }
diff --git a/lib/external/tests/external_api_test.php b/lib/external/tests/external_api_test.php
index 915831892ef9c..cc359cae2294a 100644
--- a/lib/external/tests/external_api_test.php
+++ b/lib/external/tests/external_api_test.php
@@ -82,6 +82,52 @@ public function test_validate_params(): void {
         $this->assertSame('someid', key($result));
         $this->assertSame(6, $result['someid']);
         $this->assertSame('aaa', $result['text']);
+
+        // Missing required value (an exception is thrown).
+        $testdata = [];
+        try {
+            external_api::clean_returnvalue($description, $testdata);
+            $this->fail('Exception expected');
+        } catch (\moodle_exception $ex) {
+            $this->assertInstanceOf(\invalid_response_exception::class, $ex);
+            $this->assertSame('Invalid response value detected (Error in response - '
+                . 'Missing following required key in a single structure: text)', $ex->getMessage());
+        }
+
+        // Test nullable external_value may optionally return data.
+        $description = new external_function_parameters([
+            'value' => new external_value(PARAM_INT, '', VALUE_REQUIRED, null, NULL_ALLOWED)
+        ]);
+        $testdata = ['value' => null];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = ['value' => 1];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+
+        // Test nullable external_single_structure may optionally return data.
+        $description = new external_function_parameters([
+            'value' => new external_single_structure(['value2' => new external_value(PARAM_INT)],
+                '', VALUE_REQUIRED, null, NULL_ALLOWED)
+        ]);
+        $testdata = ['value' => null];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = ['value' => ['value2' => 1]];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+
+        // Test nullable external_multiple_structure may optionally return data.
+        $description = new external_function_parameters([
+            'value' => new external_multiple_structure(
+                new external_value(PARAM_INT), '', VALUE_REQUIRED, null, NULL_ALLOWED)
+        ]);
+        $testdata = ['value' => null];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = ['value' => [1]];
+        $cleanedvalue = external_api::clean_returnvalue($description, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
     }
 
     /**
@@ -162,8 +208,56 @@ public function test_clean_returnvalue(): void {
         $singlestructure['object'] = $object;
         $singlestructure['value2'] = 'Some text';
         $testdata = [$singlestructure];
-        $this->expectException('invalid_response_exception');
+        try {
+            external_api::clean_returnvalue($returndesc, $testdata);
+            $this->fail('Exception expected');
+        } catch (\moodle_exception $ex) {
+            $this->assertInstanceOf(\invalid_response_exception::class, $ex);
+            $this->assertSame('Invalid response value detected (object => Invalid response value detected '
+                . '(Error in response - Missing following required key in a single structure: value1): Error in response - '
+                . 'Missing following required key in a single structure: value1)', $ex->getMessage());
+        }
+
+        // Fail if no data provided when value required.
+        $testdata = null;
+        try {
+            external_api::clean_returnvalue($returndesc, $testdata);
+            $this->fail('Exception expected');
+        } catch (\moodle_exception $ex) {
+            $this->assertInstanceOf(\invalid_response_exception::class, $ex);
+            $this->assertSame('Invalid response value detected (Only arrays accepted. The bad value is: \'\')',
+                $ex->getMessage());
+        }
+
+        // Test nullable external_multiple_structure may optionally return data.
+        $returndesc = new external_multiple_structure(
+            new external_value(PARAM_INT),
+            '', VALUE_REQUIRED, null, NULL_ALLOWED);
+        $testdata = null;
+        $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = [1];
+        $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+
+        // Test nullable external_single_structure may optionally return data.
+        $returndesc = new external_single_structure(['value' => new external_value(PARAM_INT)],
+            '', VALUE_REQUIRED, null, NULL_ALLOWED);
+        $testdata = null;
+        $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = ['value' => 1];
+        $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+
+        // Test nullable external_value may optionally return data.
+        $returndesc = new external_value(PARAM_INT, '', VALUE_REQUIRED, null, NULL_ALLOWED);
+        $testdata = null;
+        $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
+        $testdata = 1;
         $cleanedvalue = external_api::clean_returnvalue($returndesc, $testdata);
+        $this->assertSame($testdata, $cleanedvalue);
     }
 
     /**