diff --git a/src/MessageValidator.php b/src/MessageValidator.php
index 474aec8..4697a31 100644
--- a/src/MessageValidator.php
+++ b/src/MessageValidator.php
@@ -62,7 +62,7 @@ private static function convertLambdaMessage(Message $lambdaMessage)
      * @param string $hostNamePattern
      */
     public function __construct(
-        callable $certClient = null,
+        ?callable $certClient = null,
         $hostNamePattern = ''
     ) {
         $this->certClient = $certClient ?: function($certUrl) {