diff --git a/core/modules/field_ui/src/ConfigTranslation/EntityDisplaySubscriber.php b/core/modules/field_ui/src/ConfigTranslation/EntityDisplaySubscriber.php
index 4039c5e..1b4c737 100644
--- a/core/modules/field_ui/src/ConfigTranslation/EntityDisplaySubscriber.php
+++ b/core/modules/field_ui/src/ConfigTranslation/EntityDisplaySubscriber.php
@@ -8,7 +8,7 @@
 namespace Drupal\field_ui\ConfigTranslation;
 
 use Drupal\config_translation\ConfigEntityMapper;
-use Drupal\config_translation\Event\ConfigMapperEvent;
+use Drupal\config_translation\Event\ConfigMapperPopulateEvent;
 use Drupal\config_translation\Event\ConfigTranslationEvents;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -39,18 +39,18 @@ public function __construct(ConfigFactoryInterface $config_factory) {
   /**
    * Reacts to the population of a configuration mapper.
    *
-   * @param \Drupal\config_translation\Event\ConfigMapperEvent $event
+   * @param \Drupal\config_translation\Event\ConfigMapperPopulateEvent $event
    *   The configuration mapper event.
    *
    * @see \Drupal\field_ui\Routing\RouteSubscriber::alterRoutes()
    */
-  public function onMapperPopulate(ConfigMapperEvent $event) {
+  public function onMapperPopulate(ConfigMapperPopulateEvent $event) {
     $mapper = $event->getMapper();
     $base_route = $mapper->getBaseRoute();
 
     if (
       !($mapper instanceof ConfigEntityMapper) ||
-      !($entity_type_id = $base_route->getOption('_field_ui_base_route'))
+      !($entity_type_id = $base_route->getOption('_field_ui_base_route_for'))
     ) {
       return;
     }
@@ -59,6 +59,9 @@ public function onMapperPopulate(ConfigMapperEvent $event) {
     // Determine the bundle (if any) of the route.
     $bundle = NULL;
     if ($entity_type_id === $entity->getEntityType()->getBundleOf()) {
+      // Here, $entity is an extension of
+      // \Drupal\Core\Config\Entity\ConfigEntityBundleBase,
+      // so the entity's ID is a bundle name.
       $bundle = $entity->id();
     }
     elseif ($base_route->getDefault('bundle')) {
diff --git a/core/modules/field_ui/src/Routing/RouteSubscriber.php b/core/modules/field_ui/src/Routing/RouteSubscriber.php
index de6a286..8fce3ba 100644
--- a/core/modules/field_ui/src/Routing/RouteSubscriber.php
+++ b/core/modules/field_ui/src/Routing/RouteSubscriber.php
@@ -45,23 +45,24 @@ protected function alterRoutes(RouteCollection $collection) {
         if (!$entity_route = $collection->get($route_name)) {
           continue;
         }
+        $path = $entity_route->getPath();
+
+        $options = $entity_route->getOptions();
+
         // Set a route option to easily identify all Field UI base routes and to
         // determine the entity type the route is for.
-        $entity_route->setOption('_field_ui_base_route', $entity_type_id);
+        $entity_route->setOption('_field_ui_base_route_for', $entity_type_id);
         // RouteCollection::add() overwrites exiting routes with the same name.
         $collection->add($route_name, $entity_route);
 
-        $path = $entity_route->getPath();
-
-        $options = $entity_route->getOptions();
         if ($bundle_entity_type = $entity_type->getBundleEntityType()) {
           $options['parameters'][$bundle_entity_type] = array(
             'type' => 'entity:' . $bundle_entity_type,
           );
         }
-        // Set a route option to easily identify all Field UI base routes and to
+        // Set a route option to easily identify all Field UI routes and to
         // determine the entity type the routes are for.
-        $options['_field_ui_route'] = $entity_type_id;
+        $options['_field_ui_route_for'] = $entity_type_id;
         // This is retained for backwards-compatibility.
         // @todo Remove in Drupal 9
         $options['_field_ui'] = TRUE;