diff --git a/src/FrontMatter.php b/src/FrontMatter.php
index a6b0a0b..2bd8e83 100644
--- a/src/FrontMatter.php
+++ b/src/FrontMatter.php
@@ -39,7 +39,7 @@ public static function createJson(): self
         return new self(new JsonWithoutBracesProcessor(), '{', '}');
     }
 
-    public function __construct(ProcessorInterface $processor = null, string $startSep = '---', string $endSep = '---')
+    public function __construct(?ProcessorInterface $processor = null, string $startSep = '---', string $endSep = '---')
     {
         $this->startSep = $startSep;
         $this->endSep = $endSep;
diff --git a/src/Pug/PugCommentFrontMatter.php b/src/Pug/PugCommentFrontMatter.php
index 6bc174d..b0d47ce 100644
--- a/src/Pug/PugCommentFrontMatter.php
+++ b/src/Pug/PugCommentFrontMatter.php
@@ -29,12 +29,12 @@ private function __construct()
         // prevent any instantiation
     }
 
-    public static function createWithEndComment(ProcessorInterface $processor = null): FrontMatter
+    public static function createWithEndComment(?ProcessorInterface $processor = null): FrontMatter
     {
         return new FrontMatter($processor ?? new YamlProcessor(), '//-', '//-');
     }
 
-    public static function create(ProcessorInterface $processor = null): FrontMatter
+    public static function create(?ProcessorInterface $processor = null): FrontMatter
     {
         return new FrontMatter($processor ?? new YamlProcessor(), '//-', "\n");
     }
diff --git a/src/Twig/FrontMatterLoader.php b/src/Twig/FrontMatterLoader.php
index 7db03bf..1c20631 100644
--- a/src/Twig/FrontMatterLoader.php
+++ b/src/Twig/FrontMatterLoader.php
@@ -30,7 +30,7 @@ class FrontMatterLoader implements LoaderInterface
     public function __construct(
         FrontMatterInterface $parser,
         LoaderInterface $loader,
-        DataToTwigConvertor $convertor = null
+        ?DataToTwigConvertor $convertor = null
     )
     {
         $this->loader = $loader;
diff --git a/src/Twig/TwigCommentFrontMatter.php b/src/Twig/TwigCommentFrontMatter.php
index ca01034..a4bad1d 100644
--- a/src/Twig/TwigCommentFrontMatter.php
+++ b/src/Twig/TwigCommentFrontMatter.php
@@ -29,7 +29,7 @@ private function __construct()
         // prevent any instantiation
     }
 
-    public static function create(ProcessorInterface $processor = null): FrontMatter
+    public static function create(?ProcessorInterface $processor = null): FrontMatter
     {
         return new FrontMatter($processor ?? new YamlProcessor(), '{#---', '---#}');
     }