--- a/src/Plugin/migrate/destination/DraggableViews.php
+++ b/src/Plugin/migrate/destination/DraggableViews.php
@@ -101,7 +101,7 @@
   /**
    * {@inheritdoc}
    */
-  public function fields(MigrationInterface $migration = NULL) {
+  public function fields(?MigrationInterface $migration = NULL) {
     return [
       'dvid' => $this->t('The primary identifier'),
       'view_name' => $this->t('The view name.'),