diff --git a/SendinblueTransport.php b/SendinblueTransport.php
index 174bf49..7227666 100644
--- a/SendinblueTransport.php
+++ b/SendinblueTransport.php
@@ -33,7 +33,7 @@ final class SendinblueTransport extends AbstractTransport
     private string $apiKey;
     private string $sender;
 
-    public function __construct(#[\SensitiveParameter] string $apiKey, string $sender, HttpClientInterface $client = null, EventDispatcherInterface $dispatcher = null)
+    public function __construct(#[\SensitiveParameter] string $apiKey, string $sender, ?HttpClientInterface $client = null, ?EventDispatcherInterface $dispatcher = null)
     {
         $this->apiKey = $apiKey;
         $this->sender = $sender;
diff --git a/Tests/SendinblueTransportTest.php b/Tests/SendinblueTransportTest.php
index 5b817ad..eca1daf 100644
--- a/Tests/SendinblueTransportTest.php
+++ b/Tests/SendinblueTransportTest.php
@@ -26,7 +26,7 @@
  */
 final class SendinblueTransportTest extends TransportTestCase
 {
-    public static function createTransport(HttpClientInterface $client = null): SendinblueTransport
+    public static function createTransport(?HttpClientInterface $client = null): SendinblueTransport
     {
         return (new SendinblueTransport('api-key', '0611223344', $client ?? new MockHttpClient()))->setHost('host.test');
     }