diff --git modules/taxonomy/taxonomy.module modules/taxonomy/taxonomy.module
index d7a5e9a..c298f16 100644
--- modules/taxonomy/taxonomy.module
+++ modules/taxonomy/taxonomy.module
@@ -1304,7 +1304,12 @@ function taxonomy_field_widget_info_alter(&$info) {
  * Implements hook_options_list().
  */
 function taxonomy_options_list($field) {
-  $function = !empty($field['settings']['options_list_callback']) ? $field['settings']['options_list_callback'] : 'taxonomy_allowed_values';
+  if (!empty($field['settings']['options_list_callback']) && function_exists($field['settings']['options_list_callback'])) {
+    $function = $field['settings']['options_list_callback'];
+  }
+  else {
+    $function = 'taxonomy_allowed_values';
+  }
   return $function($field);
 }