diff --git a/src/customer/Customer.php b/src/customer/Customer.php
index 4b02c668..51658920 100755
--- a/src/customer/Customer.php
+++ b/src/customer/Customer.php
@@ -37,7 +37,7 @@ class Customer implements CustomerInterface
      */
     protected $sellers = [];
 
-    public function __construct($id, $login, CustomerInterface $seller = null)
+    public function __construct($id, $login, ?CustomerInterface $seller = null)
     {
         $this->id = $id;
         $this->login = $login;