diff --git a/src/Commands/ArtifactCommand.php b/src/Commands/ArtifactCommand.php
index 84c0277..0b79b54 100644
--- a/src/Commands/ArtifactCommand.php
+++ b/src/Commands/ArtifactCommand.php
@@ -138,8 +138,8 @@ class ArtifactCommand extends Command {
    *   Command name.
    */
   public function __construct(
-    ArtifactGit $gitWrapper = NULL,
-    Filesystem $fsFileSystem = NULL,
+    ?ArtifactGit $gitWrapper = NULL,
+    ?Filesystem $fsFileSystem = NULL,
     ?string $name = NULL,
   ) {
     parent::__construct($name);
@@ -813,7 +813,7 @@ protected function restoreLocalExclude(string $path): void {
    * @throws \Exception
    *   If removal command finished with an error.
    */
-  protected function removeIgnoredFiles(string $location, string $gitignorePath = NULL): void {
+  protected function removeIgnoredFiles(string $location, ?string $gitignorePath = NULL): void {
     $location = $this->getSourcePathGitRepository();
     $gitignorePath = $gitignorePath ?: $location . DIRECTORY_SEPARATOR . '.gitignore';
 
@@ -912,7 +912,7 @@ protected function getTokenBranch(): string {
    *
    * @throws \Exception
    */
-  protected function getTokenTags(string $delimiter = NULL): string {
+  protected function getTokenTags(?string $delimiter = NULL): string {
     $delimiter = $delimiter ?: '-';
     // We just want to get all tags point to the HEAD.
     $tags = $this
diff --git a/src/Traits/FilesystemTrait.php b/src/Traits/FilesystemTrait.php
index f306e02..a1d4f3d 100644
--- a/src/Traits/FilesystemTrait.php
+++ b/src/Traits/FilesystemTrait.php
@@ -41,7 +41,7 @@ trait FilesystemTrait {
    *
    * @throws \Exception
    */
-  protected function fsSetRootDir(string $path = NULL): void {
+  protected function fsSetRootDir(?string $path = NULL): void {
     $path = empty($path) ? $this->fsGetRootDir() : $this->fsGetAbsolutePath($path);
     $this->fsPathsExist($path);
     $this->fsRootDir = $path;
@@ -129,7 +129,7 @@ protected function fsIsCommandAvailable(string $command): bool {
    * @return string
    *   Absolute path for provided file.
    */
-  protected function fsGetAbsolutePath(string $file, string $root = NULL): string {
+  protected function fsGetAbsolutePath(string $file, ?string $root = NULL): string {
     if ($this->fsFileSystem->isAbsolutePath($file)) {
       return $this->realpath($file);
     }
diff --git a/tests/phpunit/Exception/ErrorException.php b/tests/phpunit/Exception/ErrorException.php
index a2a08bf..f4e7e43 100644
--- a/tests/phpunit/Exception/ErrorException.php
+++ b/tests/phpunit/Exception/ErrorException.php
@@ -14,7 +14,7 @@ class ErrorException extends Exception {
   /**
    * {@inheritdoc}
    */
-  public function __construct(string $message, int $code, string $file, int $line, \Exception $previous = NULL) {
+  public function __construct(string $message, int $code, string $file, int $line, ?\Exception $previous = NULL) {
     parent::__construct($message, $code, $previous);
 
     $this->file = $file;
diff --git a/tests/phpunit/Traits/CommandTrait.php b/tests/phpunit/Traits/CommandTrait.php
index e745275..2063818 100644
--- a/tests/phpunit/Traits/CommandTrait.php
+++ b/tests/phpunit/Traits/CommandTrait.php
@@ -183,7 +183,7 @@ protected function gitGetCommittedFiles(string $path): array {
    *   Optional path to the repository directory. If not provided, fixture
    *   directory is used.
    */
-  protected function gitCreateFixtureCommits(int $count, int $offset = 0, string $path = NULL): void {
+  protected function gitCreateFixtureCommits(int $count, int $offset = 0, ?string $path = NULL): void {
     $path = $path ? $path : $this->src;
     for ($i = $offset; $i < $count + $offset; $i++) {
       $this->gitCreateFixtureCommit($i + 1, $path);
@@ -202,7 +202,7 @@ protected function gitCreateFixtureCommits(int $count, int $offset = 0, string $
    * @return string
    *   Hash of created commit.
    */
-  protected function gitCreateFixtureCommit(int $index, string $path = NULL): string {
+  protected function gitCreateFixtureCommit(int $index, ?string $path = NULL): string {
     $path = $path ? $path : $this->src;
     $filename = 'f' . $index;
     $this->gitCreateFixtureFile($path, $filename);
@@ -348,7 +348,7 @@ protected function gitAddTag(string $path, string $name, bool $annotate = FALSE)
    *
    * @todo Update arguments order and add assertion message.
    */
-  protected function gitAssertFilesExist(string $path, $files, string $branch = NULL): void {
+  protected function gitAssertFilesExist(string $path, $files, ?string $branch = NULL): void {
     $files = is_array($files) ? $files : [$files];
     if ($branch) {
       $this->gitCheckout($path, $branch);
@@ -368,7 +368,7 @@ protected function gitAssertFilesExist(string $path, $files, string $branch = NU
    * @param string|null $branch
    *   Optional branch. If set, will be checked out before assertion.
    */
-  protected function gitAssertFilesNotExist(string $path, $files, string $branch = NULL): void {
+  protected function gitAssertFilesNotExist(string $path, $files, ?string $branch = NULL): void {
     $files = is_array($files) ? $files : [$files];
     if ($branch) {
       $this->gitCheckout($path, $branch);
diff --git a/tests/phpunit/Traits/ReflectionTrait.php b/tests/phpunit/Traits/ReflectionTrait.php
index adaf451..bec0658 100644
--- a/tests/phpunit/Traits/ReflectionTrait.php
+++ b/tests/phpunit/Traits/ReflectionTrait.php
@@ -89,7 +89,7 @@ protected static function setProtectedValue($object, $property, mixed $value): v
    * @return mixed
    *   Protected property value.
    */
-  protected static function getProtectedValue($object, $property) {
+  protected static function getProtectedValue($object, $property): mixed {
     $class = new \ReflectionClass($object::class);
     $property = $class->getProperty($property);
     $property->setAccessible(TRUE);