diff --git a/src/ClassMapGenerator.php b/src/ClassMapGenerator.php
index fd905c3..53f9a42 100644
--- a/src/ClassMapGenerator.php
+++ b/src/ClassMapGenerator.php
@@ -61,7 +61,7 @@ public function __construct(array $extensions = ['php', 'inc'])
      *
      * @return $this
      */
-    public function avoidDuplicateScans(FileList $scannedFiles = null): self
+    public function avoidDuplicateScans(?FileList $scannedFiles = null): self
     {
         $this->scannedFiles = $scannedFiles ?? new FileList;
 
@@ -100,7 +100,7 @@ public function getClassMap(): ClassMap
      *
      * @throws \RuntimeException When the path is neither an existing file nor directory
      */
-    public function scanPaths($path, string $excluded = null, string $autoloadType = 'classmap', ?string $namespace = null): void
+    public function scanPaths($path, ?string $excluded = null, string $autoloadType = 'classmap', ?string $namespace = null): void
     {
         if (!in_array($autoloadType, ['psr-0', 'psr-4', 'classmap'], true)) {
             throw new \InvalidArgumentException('$autoloadType must be one of: "psr-0", "psr-4" or "classmap"');
diff --git a/src/PhpFileCleaner.php b/src/PhpFileCleaner.php
index 3a74478..7c0f72a 100644
--- a/src/PhpFileCleaner.php
+++ b/src/PhpFileCleaner.php
@@ -240,7 +240,7 @@ private function peek(string $char): bool
      * @param non-empty-string $regex
      * @param null|array<int, string> $match
      */
-    private function match(string $regex, array &$match = null): bool
+    private function match(string $regex, ?array &$match = null): bool
     {
         return Preg::isMatch($regex, $this->contents, $match, 0, $this->index);
     }