diff --git a/src/Processor/FieldsProcessorPluginBase.php b/src/Processor/FieldsProcessorPluginBase.php
index 2aed3656..0c36ea61 100644
--- a/src/Processor/FieldsProcessorPluginBase.php
+++ b/src/Processor/FieldsProcessorPluginBase.php
@@ -268,7 +268,7 @@ public function validateConfigurationForm(array &$form, FormStateInterface $form
       }
     }
     else {
-      $fields = array_keys($form['#options']);
+      $fields = array_keys($form['fields']['#options']);
     }
     $form_state->setValue('fields', $fields);
   }