diff --git a/src/Exception/OAuthServerException.php b/src/Exception/OAuthServerException.php
index 264f0e87..7c2ddae0 100644
--- a/src/Exception/OAuthServerException.php
+++ b/src/Exception/OAuthServerException.php
@@ -95,7 +95,7 @@ class OAuthServerException extends \Exception
      *
      * @param ServerRequestInterface $serverRequest
      */
-    public function setServerRequest($serverRequest)
+    public function setServerRequest(ServerRequestInterface $serverRequest)
     {
         $this->serverRequest = $serverRequest;
     }
@@ -137,7 +137,7 @@ class OAuthServerException extends \Exception
      *
      * @return static
      */
-    public static function invalidClient($serverRequest)
+    public static function invalidClient(ServerRequestInterface $serverRequest)
     {
         $exception = new static('Client authentication failed', 4, 'invalid_client', 401);