diff --git a/src/MessageBird/Client.php b/src/MessageBird/Client.php
index ab84987..cc14b6e 100644
--- a/src/MessageBird/Client.php
+++ b/src/MessageBird/Client.php
@@ -147,7 +147,7 @@ class Client
      */
     protected $numbersAPIClient;
 
-    public function __construct(?string $accessKey = null, Common\HttpClient $httpClient = null, array $config = [])
+    public function __construct(?string $accessKey = null, ?Common\HttpClient $httpClient = null, array $config = [])
     {
         if ($httpClient === null) {
             $this->conversationsAPIHttpClient = new Common\HttpClient(