diff --git a/src/Helper.php b/src/Helper.php
index e3d433a..a27239a 100644
--- a/src/Helper.php
+++ b/src/Helper.php
@@ -6,7 +6,7 @@
 use Tzsk\Otp\Otp;
 
 if (! function_exists('otp')) {
-    function otp(string $directory = null): Otp
+    function otp(?string $directory = null): Otp
     {
         if ($directory) {
             $store = new Repository(new FileStore(new Filesystem(), $directory));