diff --git a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
index f0bb021..3ca2b00 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
@@ -147,18 +147,19 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
       }
 
       // Process this list so it can be used in a select, showing one label
-      // and the machine name for each field.
+      // and the machine name for each field. Also cast to string so the list
+      // can be sorted.
       $options = [];
       foreach ($fields as $machine_name => $list) {
         $list = array_unique($list);
         if (count($list) > 1 ) {
-          $options[$machine_name] = $this->t('@label|... (@machine_name)', [
+          $options[$machine_name] = (string) $this->t('@label|... [@machine_name]', [
             '@label' => array_pop($list),
             '@machine_name' => $machine_name,
           ]);
         }
         else {
-          $options[$machine_name] = $this->t('@label (@machine_name)', [
+          $options[$machine_name] = (string) $this->t('@label [@machine_name]', [
             '@label' => array_pop($list),
             '@machine_name' => $machine_name,
           ]);