diff --git a/src/Component.php b/src/Component.php
index 716e0c8f787ebbb34922d1829695cbf69cc48960..39baf20d45fb796fbc82c63bc8ba265f0ec2923c 100644
--- a/src/Component.php
+++ b/src/Component.php
@@ -17,7 +17,7 @@ class Component
         return new Component\DateTime($dateTime, $format);
     }
 
-    public static function document(string $link, string $filename = null): Component\Document
+    public static function document(string $link, ?string $filename = null): Component\Document
     {
         return new Component\Document($link, $filename);
     }