diff --git a/src/Stash/Utilities.php b/src/Stash/Utilities.php
index dc8ed961..c868bff0 100644
--- a/src/Stash/Utilities.php
+++ b/src/Stash/Utilities.php
@@ -100,7 +100,7 @@ public static function decode($data, $method)
      * @param  DriverInterface $driver
      * @return string          Path for Stash files
      */
-    public static function getBaseDirectory(DriverInterface $driver = null)
+    public static function getBaseDirectory(?DriverInterface $driver = null)
     {
         $tmp = rtrim(sys_get_temp_dir(), '/\\') . '/';