diff --git a/SwaggerGen/Parser/Php/Parser.php b/SwaggerGen/Parser/Php/Parser.php
index cb4d92e..e26a1f7 100644
--- a/SwaggerGen/Parser/Php/Parser.php
+++ b/SwaggerGen/Parser/Php/Parser.php
@@ -349,7 +349,7 @@ private function inherit(Entity\ParserClass $Class)
      * @throws Exception
      * @throws Exception
      */
-    private function expand(array $Statements, ParserClass $Self = null)
+    private function expand(array $Statements, ?ParserClass $Self = null)
     {
         $output = [];
 
diff --git a/SwaggerGen/Swagger/AbstractObject.php b/SwaggerGen/Swagger/AbstractObject.php
index f095b59..d718662 100644
--- a/SwaggerGen/Swagger/AbstractObject.php
+++ b/SwaggerGen/Swagger/AbstractObject.php
@@ -39,7 +39,7 @@ abstract class AbstractObject
      */
     private $extensions = array();
 
-    public function __construct(AbstractObject $parent = null)
+    public function __construct(?AbstractObject $parent = null)
     {
         $this->parent = $parent;
     }
diff --git a/SwaggerGen/Swagger/Operation.php b/SwaggerGen/Swagger/Operation.php
index fa969a4..cafc340 100644
--- a/SwaggerGen/Swagger/Operation.php
+++ b/SwaggerGen/Swagger/Operation.php
@@ -44,7 +44,7 @@ public function getConsumes(): array
     /**
      * @param string $summary
      */
-    public function __construct(AbstractObject $parent, $summary = null, Tag $tag = null)
+    public function __construct(AbstractObject $parent, $summary = null, ?Tag $tag = null)
     {
         parent::__construct($parent);
         $this->summary = $summary;
diff --git a/SwaggerGen/Swagger/Path.php b/SwaggerGen/Swagger/Path.php
index 8815f13..ede0a41 100644
--- a/SwaggerGen/Swagger/Path.php
+++ b/SwaggerGen/Swagger/Path.php
@@ -36,7 +36,7 @@ class Path extends AbstractObject
      */
     private $tag;
 
-    public function __construct(AbstractObject $parent, Tag $Tag = null)
+    public function __construct(AbstractObject $parent, ?Tag $Tag = null)
     {
         parent::__construct($parent);
         $this->tag = $Tag;