diff --git a/inc/DAVResource.php b/inc/DAVResource.php
index a6509112..f15bfc6f 100644
--- a/inc/DAVResource.php
+++ b/inc/DAVResource.php
@@ -173,7 +173,7 @@ class DAVResource
   * field of the resource is populated with the given data, so it does not need
   * to be queried again later
   */
-  function __construct( $parameters = null, DAVResource $prefetched_collection = null ) {
+  function __construct( $parameters = null, ?DAVResource $prefetched_collection = null ) {
     $this->exists        = null;
     $this->bound_from    = null;
     $this->dav_name      = null;