diff --git a/README.md b/README.md
index 4d5fd215..85a5c590 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ The following versions of PHP are supported:
 * PHP 7.1
 * PHP 7.2
 
-The `openssl` extension is also required.
+The `openssl` and `json` extensions are also required.
 
 All HTTP messages passed to the server should be [PSR-7 compliant](https://www.php-fig.org/psr/psr-7/). This ensures interoperability with other packages and frameworks.
 
diff --git a/composer.json b/composer.json
index 48a95701..f8a2160f 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,8 @@
         "league/event": "^2.1",
         "lcobucci/jwt": "^3.2.2",
         "psr/http-message": "^1.0.1",
-        "defuse/php-encryption": "^2.1"
+        "defuse/php-encryption": "^2.1",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.3 || ^7.0",
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);