diff --git a/src/Storyblok/BaseClient.php b/src/Storyblok/BaseClient.php
index 465566e..aa0a3d0 100644
--- a/src/Storyblok/BaseClient.php
+++ b/src/Storyblok/BaseClient.php
@@ -115,7 +115,7 @@ public function retryDecider(): \Closure
             $retries,
             $request,
             $response = null,
-            TransferException $exception = null
+            ?TransferException $exception = null
         ) {
             // Limit the number of retries
             if ($retries >= $this->maxRetries) {