diff --git a/src/Lexer.php b/src/Lexer.php
index 8791030..80aac02 100644
--- a/src/Lexer.php
+++ b/src/Lexer.php
@@ -13,7 +13,7 @@ class Lexer
     /**
      * @param SubLexerInterface $subLexer
      */
-    public function __construct(SubLexerInterface $subLexer = null)
+    public function __construct(?SubLexerInterface $subLexer = null)
     {
         $this->subLexer = $subLexer ?: static::createDefaultSubLexer();
     }
diff --git a/src/Parser.php b/src/Parser.php
index 3fb80dc..aa86e90 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -15,7 +15,7 @@ class Parser
     /**
      * @param NodeParserInterface $nodeParser
      */
-    public function __construct(NodeParserInterface $nodeParser = null)
+    public function __construct(?NodeParserInterface $nodeParser = null)
     {
         $this->nodeParser = $nodeParser ?: static::createDefaultNodeParser();
     }