diff --git a/library/Businessprocess/Node.php b/library/Businessprocess/Node.php
index 0510859..c95295b 100644
--- a/library/Businessprocess/Node.php
+++ b/library/Businessprocess/Node.php
@@ -98,7 +98,7 @@ abstract class Node
     /**
      * This node's icon
      *
-     * @var string
+     * @var ?string
      */
     protected $icon;