diff --git a/core/modules/options/lib/Drupal/options/Plugin/field/field_type/ListItemBase.php b/core/modules/options/lib/Drupal/options/Plugin/field/field_type/ListItemBase.php
index d6813b0..bc46a05 100644
--- a/core/modules/options/lib/Drupal/options/Plugin/field/field_type/ListItemBase.php
+++ b/core/modules/options/lib/Drupal/options/Plugin/field/field_type/ListItemBase.php
@@ -21,7 +21,10 @@
    * {@inheritdoc}
    */
   public function getPossibleValues(AccountInterface $account = NULL) {
-    return array_keys($this->getPossibleOptions($account));
+    // Flatten options firstly, because Possible Options may contain group
+    // arrays.
+    $flatten_options = $this->flattenOptions($this->getPossibleOptions($account));
+    return array_keys($flatten_options);
   }
 
   /**