diff --git a/src/Responses/TextToSpeechResponse.php b/src/Responses/TextToSpeechResponse.php
index f742c40..15f9ea4 100644
--- a/src/Responses/TextToSpeechResponse.php
+++ b/src/Responses/TextToSpeechResponse.php
@@ -14,7 +14,7 @@ public function __construct(ResponseInterface $response)
         $this->response = $response;
     }
 
-    public function getResponse()
+    public function getResponse(): ResponseInterface
     {
         return $this->response;
     }