diff --git a/src/Ftp/FtpAdapter.php b/src/Ftp/FtpAdapter.php
index 304417fb8..fd5d169a1 100644
--- a/src/Ftp/FtpAdapter.php
+++ b/src/Ftp/FtpAdapter.php
@@ -80,7 +80,10 @@ class FtpAdapter implements FilesystemAdapter
      */
     private $mimeTypeDetector;
 
-    private ?string $rootDirectory = null;
+    /**
+     * @var null|string
+     */
+    private $rootDirectory = null;
 
     public function __construct(
         FtpConnectionOptions $connectionOptions,