diff --git a/src/Controller/GenericRevisionController.php b/src/Controller/GenericRevisionController.php
index 26df1c0..44d90b5 100644
--- a/src/Controller/GenericRevisionController.php
+++ b/src/Controller/GenericRevisionController.php
@@ -337,13 +337,13 @@ class GenericRevisionController extends EntityComparisonBase {
    *   Vid of the left revision.
    * @param $right_vid
    *   Vid of the right revision.
-   * @param boolean $raw_plain
-   *   Flag to determinate if the route should add a raw_plain filter.
+   * @param $raw_plain
+   *   (optional) Flag to determinate if the route adds a raw-plain filter.
    *
    * @return \Drupal\Core\Url
    *   The URL object.
    */
-  protected function diffRoute(EntityInterface $entity, $left_vid, $right_vid, $raw_plain = FALSE) {
+  protected function diffRoute(EntityInterface $entity, $left_vid, $right_vid, $raw_plain = NULL) {
     $entity_type_id = $entity->getEntityTypeId();
     // @todo Remove the diff.revisions_diff route so we avoid adding extra cases.
     if ($entity->getEntityTypeId() == 'node') {