diff --git a/src/Client.php b/src/Client.php
index 9d8d2dd..be3d41b 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -65,9 +65,9 @@ public function __construct(
         string $endpointUrl,
         array $authorizationHeaders = [],
         array $httpOptions = [],
-        ClientInterface $httpClient = null,
+        ?ClientInterface $httpClient = null,
         string $requestMethod = 'POST',
-        AuthInterface $auth = null
+        ?AuthInterface $auth = null
     ) {
         $headers = array_merge(
             $authorizationHeaders,