diff --git a/src/Api/Projects.php b/src/Api/Projects.php
index 3f5d1962..1e2f2ba1 100644
--- a/src/Api/Projects.php
+++ b/src/Api/Projects.php
@@ -466,7 +466,7 @@ public function pipelineTestReportSummary($project_id, int $pipeline_id)
      *
      * @return mixed
      */
-    public function createPipeline($project_id, string $commit_ref, array $variables = null)
+    public function createPipeline($project_id, string $commit_ref, ?array $variables = null)
     {
         $parameters = [];
 
diff --git a/src/Api/Repositories.php b/src/Api/Repositories.php
index 096f69a2..43d466f6 100644
--- a/src/Api/Repositories.php
+++ b/src/Api/Repositories.php
@@ -419,7 +419,7 @@ public function postCommitBuildStatus($project_id, string $sha, string $state, a
      *
      * @return mixed
      */
-    public function compare($project_id, string $fromShaOrMaster, string $toShaOrMaster, bool $straight = false, string $fromProjectId = null)
+    public function compare($project_id, string $fromShaOrMaster, string $toShaOrMaster, bool $straight = false, ?string $fromProjectId = null)
     {
         $params = [
             'from' => $fromShaOrMaster,
diff --git a/src/Client.php b/src/Client.php
index 4082f1ac..cd24dcb1 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -123,7 +123,7 @@ class Client
      *
      * @return void
      */
-    public function __construct(Builder $httpClientBuilder = null)
+    public function __construct(?Builder $httpClientBuilder = null)
     {
         $this->httpClientBuilder = $builder = $httpClientBuilder ?? new Builder();
         $this->responseHistory = new History();
@@ -425,7 +425,7 @@ public function wiki(): Wiki
      *
      * @return void
      */
-    public function authenticate(string $token, string $authMethod, string $sudo = null): void
+    public function authenticate(string $token, string $authMethod, ?string $sudo = null): void
     {
         $this->getHttpClientBuilder()->removePlugin(Authentication::class);
         $this->getHttpClientBuilder()->addPlugin(new Authentication($authMethod, $token, $sudo));
diff --git a/src/HttpClient/Builder.php b/src/HttpClient/Builder.php
index 594b92cd..00fe2b4d 100644
--- a/src/HttpClient/Builder.php
+++ b/src/HttpClient/Builder.php
@@ -100,10 +100,10 @@ final class Builder
      * @return void
      */
     public function __construct(
-        ClientInterface $httpClient = null,
-        RequestFactoryInterface $requestFactory = null,
-        StreamFactoryInterface $streamFactory = null,
-        UriFactoryInterface $uriFactory = null
+        ?ClientInterface $httpClient = null,
+        ?RequestFactoryInterface $requestFactory = null,
+        ?StreamFactoryInterface $streamFactory = null,
+        ?UriFactoryInterface $uriFactory = null
     ) {
         $this->httpClient = $httpClient ?? Psr18ClientDiscovery::find();
         $this->requestFactory = $requestFactory ?? Psr17FactoryDiscovery::findRequestFactory();
diff --git a/src/HttpClient/Plugin/Authentication.php b/src/HttpClient/Plugin/Authentication.php
index 2160f5cd..39cd0b79 100644
--- a/src/HttpClient/Plugin/Authentication.php
+++ b/src/HttpClient/Plugin/Authentication.php
@@ -43,7 +43,7 @@ final class Authentication implements Plugin
      *
      * @return void
      */
-    public function __construct(string $method, string $token, string $sudo = null)
+    public function __construct(string $method, string $token, ?string $sudo = null)
     {
         $this->headers = self::buildHeaders($method, $token, $sudo);
     }
@@ -77,7 +77,7 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
      *
      * @return array<string,string>
      */
-    private static function buildHeaders(string $method, string $token, string $sudo = null): array
+    private static function buildHeaders(string $method, string $token, ?string $sudo = null): array
     {
         $headers = [];
 
diff --git a/src/ResultPager.php b/src/ResultPager.php
index 5c22d795..d0618302 100644
--- a/src/ResultPager.php
+++ b/src/ResultPager.php
@@ -66,7 +66,7 @@ final class ResultPager implements ResultPagerInterface
      *
      * @return void
      */
-    public function __construct(Client $client, int $perPage = null)
+    public function __construct(Client $client, ?int $perPage = null)
     {
         if (null !== $perPage && ($perPage < 1 || $perPage > 100)) {
             throw new ValueError(\sprintf('%s::__construct(): Argument #2 ($perPage) must be between 1 and 100, or null', self::class));