diff --git a/functions.php b/functions.php
index 38dcb91..4e13e01 100644
--- a/functions.php
+++ b/functions.php
@@ -29,7 +29,7 @@ function serialize($data)
  * @param array|null $options
  * @return mixed
  */
-function unserialize($data, array $options = null)
+function unserialize($data, ?array $options = null)
 {
     SerializableClosure::enterContext();
     $data = ($options === null || \PHP_MAJOR_VERSION < 7)