diff --git a/src/Support/UndefinedValue.php b/src/Support/UndefinedValue.php
index 74b2663a..523210ae 100644
--- a/src/Support/UndefinedValue.php
+++ b/src/Support/UndefinedValue.php
@@ -5,7 +5,7 @@
 /**
  * @internal
  */
-class UndefinedValue implements \JsonSerializable
+final class UndefinedValue implements \JsonSerializable
 {
     /**
      * Determine if value is equivalent to "undefined" or "null".