diff --git a/classes/option/field_base.php b/classes/option/field_base.php
index 46087a00e..ef1aca1ff 100644
--- a/classes/option/field_base.php
+++ b/classes/option/field_base.php
@@ -191,7 +191,7 @@ public static function get_subfields() {
      * @param stdClass|null $formdata
      * @param field_base|null $myself
      * @param mixed $mockdata // Only needed if there the object needs params for the save_data function.
-     * @param string $key
+     * @param string|null $key
      * @param mixed $value
      *
      * @return array
@@ -201,7 +201,7 @@ public function check_for_changes(
         ?stdClass $formdata = null,
         ?field_base $myself = null,
         mixed $mockdata = '',
-        string $key = '',
+        ?string $key = null,
         mixed $value = ''): array {
         $excludeclassesfromtrackingchanges = [
             'addtocalendar',