diff --git a/rector.php b/rector.php
index 7e97cb42..b389625a 100644
--- a/rector.php
+++ b/rector.php
@@ -10,10 +10,12 @@
  */
 
 use Rector\Config\RectorConfig;
+use Rector\Php84\Rector\Param\ExplicitNullableParamTypeRector;
 use Rector\PHPUnit\CodeQuality\Rector\Class_\PreferPHPUnitThisCallRector;
 use Rector\PHPUnit\CodeQuality\Rector\ClassMethod\ReplaceTestAnnotationWithPrefixedFunctionRector;
 use Rector\PHPUnit\Set\PHPUnitSetList;
 use Rector\Set\ValueObject\LevelSetList;
+use Rector\ValueObject\PhpVersion;
 
 return static function (RectorConfig $rectorConfig): void {
     $rectorConfig->importNames();
@@ -32,4 +34,8 @@
         PHPUnitSetList::PHPUNIT_90,
         LevelSetList::UP_TO_PHP_74,
     ]);
+    $rectorConfig->rules([
+        ExplicitNullableParamTypeRector::class,
+    ]);
+    $rectorConfig->phpVersion(PhpVersion::PHP_84);
 };
diff --git a/src/Cli/Symfony/ConsoleApplication.php b/src/Cli/Symfony/ConsoleApplication.php
index 412de658..68edfda5 100644
--- a/src/Cli/Symfony/ConsoleApplication.php
+++ b/src/Cli/Symfony/ConsoleApplication.php
@@ -32,7 +32,7 @@ public function __construct(FactoryInterface $factory, OutputInterface $output,
         $this->output = $output;
     }
 
-    public function run(InputInterface $input = null, OutputInterface $output = null): int
+    public function run(?InputInterface $input = null, ?OutputInterface $output = null): int
     {
         return parent::run($input, $this->output);
     }
diff --git a/src/Domain/Clock/ClockInterface.php b/src/Domain/Clock/ClockInterface.php
index 09cf86e6..013590d5 100644
--- a/src/Domain/Clock/ClockInterface.php
+++ b/src/Domain/Clock/ClockInterface.php
@@ -15,7 +15,7 @@ interface ClockInterface
 {
     public function currentTime(): int;
 
-    public function stringToTime(string $string, int $time = null): int;
+    public function stringToTime(string $string, ?int $time = null): int;
 
     public function createTimestampFromFormat(string $format, string $time): int;
 }
diff --git a/src/Domain/Clock/SystemClock.php b/src/Domain/Clock/SystemClock.php
index 24b1acc1..6cc5f10e 100644
--- a/src/Domain/Clock/SystemClock.php
+++ b/src/Domain/Clock/SystemClock.php
@@ -20,7 +20,7 @@ public function currentTime(): int
         return time();
     }
 
-    public function stringToTime(string $string, int $time = null): int
+    public function stringToTime(string $string, ?int $time = null): int
     {
         $time ??= $this->currentTime();
         $timestamp = strtotime($string, $time);
diff --git a/src/Domain/Model/Deployment.php b/src/Domain/Model/Deployment.php
index 43d5499b..e1502a06 100644
--- a/src/Domain/Model/Deployment.php
+++ b/src/Domain/Model/Deployment.php
@@ -101,7 +101,7 @@ class Deployment implements LoggerAwareInterface
     private string $deploymentLockIdentifier;
     private ContainerInterface $container;
 
-    public function __construct(ContainerInterface $container, string $name, string $deploymentLockIdentifier = null)
+    public function __construct(ContainerInterface $container, string $name, ?string $deploymentLockIdentifier = null)
     {
         $this->container = $container;
         $this->name = $name;
diff --git a/src/Domain/Model/Workflow.php b/src/Domain/Model/Workflow.php
index 8b48ff7f..bb660658 100644
--- a/src/Domain/Model/Workflow.php
+++ b/src/Domain/Model/Workflow.php
@@ -51,7 +51,7 @@ abstract public function getName(): string;
     /**
      * Remove the given task from all stages and applications
      */
-    public function removeTask(string $removeTask, Application $application = null): self
+    public function removeTask(string $removeTask, ?Application $application = null): self
     {
         $removeApplicationName = $application instanceof Application ? $application->getName() : null;
 
@@ -111,7 +111,7 @@ public function forStage(string $stage, $tasks): self
      * @param string $stage The name of the stage when this task shall be executed
      * @param string $step A stage has three steps "before", "tasks" and "after"
      */
-    protected function addTaskToStage($tasks, string $stage, Application $application = null, string $step = 'tasks'): void
+    protected function addTaskToStage($tasks, string $stage, ?Application $application = null, string $step = 'tasks'): void
     {
         if (!is_array($tasks)) {
             $tasks = [$tasks];
@@ -137,7 +137,7 @@ protected function addTaskToStage($tasks, string $stage, Application $applicatio
      *
      * @return Workflow
      */
-    public function addTask($tasks, string $stage, Application $application = null)
+    public function addTask($tasks, string $stage, ?Application $application = null)
     {
         $this->addTaskToStage($tasks, $stage, $application);
 
@@ -151,7 +151,7 @@ public function addTask($tasks, string $stage, Application $application = null)
      *
      * @param array<int, class-string|string>|string $tasks
      */
-    public function afterTask(string $task, $tasks, Application $application = null): self
+    public function afterTask(string $task, $tasks, ?Application $application = null): self
     {
         if (!is_array($tasks)) {
             $tasks = [$tasks];
@@ -174,7 +174,7 @@ public function afterTask(string $task, $tasks, Application $application = null)
      *
      * @param array<int, class-string|string>|string $tasks
      */
-    public function beforeTask(string $task, $tasks, Application $application = null): self
+    public function beforeTask(string $task, $tasks, ?Application $application = null): self
     {
         if (!is_array($tasks)) {
             $tasks = [$tasks];
@@ -209,7 +209,7 @@ public function defineTask(string $taskName, string $baseTask, array $options):
      *
      * @param class-string[]|string $tasks
      */
-    public function beforeStage(string $stage, $tasks, Application $application = null): self
+    public function beforeStage(string $stage, $tasks, ?Application $application = null): self
     {
         $this->addTaskToStage($tasks, $stage, $application, 'before');
 
@@ -221,7 +221,7 @@ public function beforeStage(string $stage, $tasks, Application $application = nu
      *
      * @param class-string[]|string $tasks
      */
-    public function afterStage(string $stage, $tasks, Application $application = null): self
+    public function afterStage(string $stage, $tasks, ?Application $application = null): self
     {
         $this->addTaskToStage($tasks, $stage, $application, 'after');
 
diff --git a/src/Integration/Factory.php b/src/Integration/Factory.php
index 58a6fc06..e00d8783 100644
--- a/src/Integration/Factory.php
+++ b/src/Integration/Factory.php
@@ -40,7 +40,7 @@ public function __construct(ContainerInterface $container, FilesystemInterface $
         $this->logger = $logger;
     }
 
-    public function getDeployment(string $deploymentName, string $configurationPath = null, bool $simulateDeployment = true, bool $initialize = true, bool $forceDeployment = false): Deployment
+    public function getDeployment(string $deploymentName, ?string $configurationPath = null, bool $simulateDeployment = true, bool $initialize = true, bool $forceDeployment = false): Deployment
     {
         $deployment = $this->createDeployment($deploymentName, $configurationPath);
 
@@ -65,7 +65,7 @@ public function getDeployment(string $deploymentName, string $configurationPath
     /**
      * @inheritDoc
      */
-    public function getDeploymentNames(string $path = null): array
+    public function getDeploymentNames(?string $path = null): array
     {
         $path = $this->getDeploymentsBasePath($path);
         $files = $this->filesystem->glob(Files::concatenatePaths([$path, '*.php']));
@@ -76,7 +76,7 @@ public function getDeploymentNames(string $path = null): array
     /**
      * @inheritDoc
      */
-    public function getDeploymentsBasePath(string $path = null): string
+    public function getDeploymentsBasePath(?string $path = null): string
     {
         $localDeploymentDescription = $this->filesystem->getRealPath('./.surf');
         if (! $path && $this->filesystem->isDirectory($localDeploymentDescription)) {
@@ -91,7 +91,7 @@ public function getDeploymentsBasePath(string $path = null): string
     /**
      * @inheritDoc
      */
-    public function getWorkspacesBasePath(string $path = null): string
+    public function getWorkspacesBasePath(?string $path = null): string
     {
         $workspacesBasePath = getenv('SURF_WORKSPACE');
 
@@ -124,7 +124,7 @@ public function getWorkspacesBasePath(string $path = null): string
      * The script has access to a deployment object as "$deployment". This could change
      * in the future.
      */
-    protected function createDeployment(string $deploymentName, string $path = null): Deployment
+    protected function createDeployment(string $deploymentName, ?string $path = null): Deployment
     {
         $deploymentConfigurationPath = $this->getDeploymentsBasePath($path);
         $workspacesBasePath = $this->getWorkspacesBasePath();
diff --git a/src/Integration/FactoryInterface.php b/src/Integration/FactoryInterface.php
index 33f2a0f2..a6b8d859 100644
--- a/src/Integration/FactoryInterface.php
+++ b/src/Integration/FactoryInterface.php
@@ -15,7 +15,7 @@
 
 interface FactoryInterface
 {
-    public function getDeployment(string $deploymentName, string $configurationPath = null, bool $simulateDeployment = true, bool $initialize = true, bool $forceDeployment = false): Deployment;
+    public function getDeployment(string $deploymentName, ?string $configurationPath = null, bool $simulateDeployment = true, bool $initialize = true, bool $forceDeployment = false): Deployment;
 
     /**
      * Get available deployment names
@@ -24,17 +24,17 @@ public function getDeployment(string $deploymentName, string $configurationPath
      *
      * @return string[]
      */
-    public function getDeploymentNames(string $path = null): array;
+    public function getDeploymentNames(?string $path = null): array;
 
     /**
      * Get the root path of the surf deployment declarations
      *
      * This defaults to ./.surf if a NULL path is given.
      */
-    public function getDeploymentsBasePath(string $path = null): string;
+    public function getDeploymentsBasePath(?string $path = null): string;
 
     /**
      * Get the base path to local workspaces
      */
-    public function getWorkspacesBasePath(string $path = null): string;
+    public function getWorkspacesBasePath(?string $path = null): string;
 }
diff --git a/tests/Unit/Domain/Service/ShellCommandServiceTest.php b/tests/Unit/Domain/Service/ShellCommandServiceTest.php
index 55804447..430806ac 100644
--- a/tests/Unit/Domain/Service/ShellCommandServiceTest.php
+++ b/tests/Unit/Domain/Service/ShellCommandServiceTest.php
@@ -34,10 +34,10 @@ class ShellCommandServiceTest extends TestCase
      */
     public function executeRemoteCommandRespectsOptionsInSshCommand(
         string $expectedCommandArguments,
-        string $username = null,
-        string $password = null,
-        int $port = null,
-        string $privateKey = null
+        ?string $username = null,
+        ?string $password = null,
+        ?int $port = null,
+        ?string $privateKey = null
     ): void {
         /** @var MockObject|ShellCommandService $service */
         $service = $this->createPartialMock(ShellCommandService::class, ['executeProcess']);