diff --git a/src/SelectTree.php b/src/SelectTree.php
index 5418f54..a9828ce 100644
--- a/src/SelectTree.php
+++ b/src/SelectTree.php
@@ -37,7 +37,7 @@ class SelectTree extends Field implements HasAffixActions
 
     protected bool $independent = true;
 
-    protected string $customKey = null;
+    protected ?string $customKey = null;
 
     protected string $titleAttribute;