diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 00f1409..1c7bf20 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -63,9 +63,9 @@ function file_field_formatter_info_alter(&$info) {
   $info['file_rss_enclosure']['field_types'][] = 'image';
 
   // Allow entity reference formatters to be used on file fields.
-  $info['entity_reference_entity_id']['field_types'] += ['file'];
-  $info['entity_reference_entity_view']['field_types'] += ['file'];
-  $info['entity_reference_label']['field_types'] += ['file'];
+  $info['entity_reference_entity_id']['field_types'][] = 'file';
+  $info['entity_reference_entity_view']['field_types'][] = 'file';
+  $info['entity_reference_label']['field_types'][] = 'file';
 }
 
 /**