diff --git a/src/AmazonAlexaService.php b/src/AmazonAlexaService.php
index be3afee..e9146bc 100644
--- a/src/AmazonAlexaService.php
+++ b/src/AmazonAlexaService.php
@@ -57,8 +57,8 @@ public function __construct(string $alexaSkillAppId, SerializerInterface $serial
      * @throws ValidationException
      */
     public function validateIncomingRequest(
-        string $signatureCertChainUrl,
-        string $signature,
+        ?string $signatureCertChainUrl,
+        ?string $signature,
         string $rawJsonRequest,
         Request $alexaRequest
     ): bool {