diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index be7bf574..b5e56831 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -76,7 +76,7 @@ class kolab_storage_cache
     /**
      * Default constructor
      */
-    public function __construct(kolab_storage_folder $storage_folder = null)
+    public function __construct(?kolab_storage_folder $storage_folder = null)
     {
         $rcmail = rcube::get_instance();
         $this->db = $rcmail->get_dbh();