diff --git a/src/Support/helpers.php b/src/Support/helpers.php
index 1c97fcd3..7a78f1dc 100644
--- a/src/Support/helpers.php
+++ b/src/Support/helpers.php
@@ -233,7 +233,7 @@ function get_unaccessible_property($object, string $property)
      * @param  ConfigInterface|null  $config
      * @return void
      */
-    function fix_file($path, ConfigInterface $config = null)
+    function fix_file($path, ?ConfigInterface $config = null)
     {
         if (is_null($config)) {
             $config = require lit_vendor_path('fixer/.php_cs_config');