diff --git a/field_hidden.module b/field_hidden.module
index 764d613a3051ab6b536e4ab7f929acc3f2c198d8..a9ff898b1f9a559636d73d193e57431eed786e5a 100644
--- a/field_hidden.module
+++ b/field_hidden.module
@@ -10,7 +10,7 @@ use Drupal\Core\Routing\RouteMatchInterface;
 /**
  * Implements hook_help().
  */
-function field_hidden_help($route_name, RouteMatchInterface $route_match = NULL) {
+function field_hidden_help($route_name, ?RouteMatchInterface $route_match = NULL) {
   switch ($route_name) {
     case 'help.page.field_hidden':
       $output = '';