diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index 5b02fa2..346ef42 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -1335,8 +1335,13 @@ function taxonomy_field_widget_info_alter(&$info) {
  * Implements hook_options_list().
  */
 function taxonomy_options_list($field, $instance) {
-  $function = !empty($field['settings']['options_list_callback']) ? $field['settings']['options_list_callback'] : 'taxonomy_allowed_values';
-  return $function($field);
+  if (!empty($field['settings']['options_list_callback'])) {
+    $function = $field['settings']['options_list_callback'];
+    return call_user_func($function, $field);
+  }
+  else {
+    return taxonomy_allowed_values($field);
+  }
 }
 
 /**