diff --git a/Node/ElementNode.php b/Node/ElementNode.php
index fbf8ea0..a188197 100644
--- a/Node/ElementNode.php
+++ b/Node/ElementNode.php
@@ -26,7 +26,7 @@ class ElementNode extends AbstractNode
     private $namespace;
     private $element;
 
-    public function __construct(string $namespace = null, string $element = null)
+    public function __construct(?string $namespace = null, ?string $element = null)
     {
         $this->namespace = $namespace;
         $this->element = $element;
diff --git a/Node/SelectorNode.php b/Node/SelectorNode.php
index 6e52b2f..cdb0e46 100644
--- a/Node/SelectorNode.php
+++ b/Node/SelectorNode.php
@@ -26,7 +26,7 @@ class SelectorNode extends AbstractNode
     private $tree;
     private $pseudoElement;
 
-    public function __construct(NodeInterface $tree, string $pseudoElement = null)
+    public function __construct(NodeInterface $tree, ?string $pseudoElement = null)
     {
         $this->tree = $tree;
         $this->pseudoElement = $pseudoElement ? strtolower($pseudoElement) : null;
diff --git a/Parser/Parser.php b/Parser/Parser.php
index d73489e..b0b6427 100644
--- a/Parser/Parser.php
+++ b/Parser/Parser.php
@@ -29,7 +29,7 @@ class Parser implements ParserInterface
 {
     private $tokenizer;
 
-    public function __construct(Tokenizer $tokenizer = null)
+    public function __construct(?Tokenizer $tokenizer = null)
     {
         $this->tokenizer = $tokenizer ?? new Tokenizer();
     }
diff --git a/XPath/Translator.php b/XPath/Translator.php
index 8ce4730..3d3ac7a 100644
--- a/XPath/Translator.php
+++ b/XPath/Translator.php
@@ -48,7 +48,7 @@ class Translator implements TranslatorInterface
     private $pseudoClassTranslators = [];
     private $attributeMatchingTranslators = [];
 
-    public function __construct(ParserInterface $parser = null)
+    public function __construct(?ParserInterface $parser = null)
     {
         $this->mainParser = $parser ?? new Parser();