diff --git a/core/includes/form.inc b/core/includes/form.inc
index 02e36fb..537eff8 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -79,8 +79,8 @@ function template_preprocess_select(&$variables) {
  *   and is not intended to be passed in to the initial function call.
  *
  * @return array
- *   A structured array of options and optgroups for use in a select form
- *   element.
+ *   A structured, possibly nested, array of options and optgroups for use in a
+ *   select form element.
  *   - label: A translated string whose value is the text of a single HTML
  *     option element, or the label attribute for an optgroup.
  *   - options: Optional, array of options for an optgroup.
@@ -113,8 +113,7 @@ function form_select_options($element, $choices = NULL) {
       ];
     }
     elseif (is_object($choice) && isset($choice->option)) {
-      $option = form_select_options($element, $choice->option);
-      $options[] = reset($option);
+      $options = array_merge($options, form_select_options($element, $choice->option));
     }
     else {
       $option = [];