diff --git a/QuickPay/API/Exceptions/TimeoutException.php b/QuickPay/API/Exceptions/TimeoutException.php
index 26dcad7..0e9e5f6 100644
--- a/QuickPay/API/Exceptions/TimeoutException.php
+++ b/QuickPay/API/Exceptions/TimeoutException.php
@@ -4,7 +4,7 @@
 
 class TimeoutException extends GenericException
 {
-    public function __construct(string $message, int $code = 0, self $previous = null)
+    public function __construct(string $message, int $code = 0, ?self $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }