diff --git a/src/CrawlerDetect.php b/src/CrawlerDetect.php
index b9d9eab..9a047fa 100644
--- a/src/CrawlerDetect.php
+++ b/src/CrawlerDetect.php
@@ -76,7 +76,7 @@ class CrawlerDetect
     /**
      * Class constructor.
      */
-    public function __construct(array $headers = null, $userAgent = null)
+    public function __construct(?array $headers = null, $userAgent = null)
     {
         $this->crawlers = new Crawlers();
         $this->exclusions = new Exclusions();