diff --git a/conformance/src/RpTest/AbstractRpTest.php b/conformance/src/RpTest/AbstractRpTest.php
index a548d74..fe9201a 100644
--- a/conformance/src/RpTest/AbstractRpTest.php
+++ b/conformance/src/RpTest/AbstractRpTest.php
@@ -47,7 +47,7 @@ public function getContainer(): ContainerInterface
         return $this->container;
     }
 
-    public function registerClient(TestInfo $testInfo, array $metadata = [], JWKSet $jwks = null): ClientInterface
+    public function registerClient(TestInfo $testInfo, array $metadata = [], ?JWKSet $jwks = null): ClientInterface
     {
         $issuer = (new IssuerBuilder())
             ->build($testInfo->getRpUri() . '/' . $this->getTestId() . '/.well-known/openid-configuration');
diff --git a/src/Exception/OAuth2Exception.php b/src/Exception/OAuth2Exception.php
index 8706cec..5eb3cf9 100644
--- a/src/Exception/OAuth2Exception.php
+++ b/src/Exception/OAuth2Exception.php
@@ -46,7 +46,7 @@ public static function isOAuth2Error(array $data): bool
     /**
      * @throws RemoteException
      */
-    public static function fromResponse(ResponseInterface $response, Throwable $previous = null): self
+    public static function fromResponse(ResponseInterface $response, ?Throwable $previous = null): self
     {
         try {
             /** @psalm-var false|array{error?: string, error_description?: string, error_uri?: string}  $data */
@@ -67,7 +67,7 @@ public static function fromResponse(ResponseInterface $response, Throwable $prev
      *
      * @psalm-param OAuth2ErrorType $params
      */
-    public static function fromParameters(array $params, Throwable $previous = null): self
+    public static function fromParameters(array $params, ?Throwable $previous = null): self
     {
         if (! static::isOAuth2Error($params)) {
             throw new InvalidArgumentException('Invalid OAuth2 exception', 0, $previous);
@@ -87,7 +87,7 @@ public function __construct(
         ?string $description = null,
         ?string $errorUri = null,
         int $code = 0,
-        Throwable $previous = null
+        ?Throwable $previous = null
     ) {
         $message = $error;
         if (null !== $description) {
diff --git a/src/Exception/RemoteException.php b/src/Exception/RemoteException.php
index a67cab5..38436fe 100644
--- a/src/Exception/RemoteException.php
+++ b/src/Exception/RemoteException.php
@@ -12,7 +12,7 @@ class RemoteException extends RuntimeException
     /** @var ResponseInterface */
     private $response;
 
-    public function __construct(ResponseInterface $response, ?string $message = null, Throwable $previous = null)
+    public function __construct(ResponseInterface $response, ?string $message = null, ?Throwable $previous = null)
     {
         parent::__construct(
             $message ?? $response->getReasonPhrase(),