diff --git a/includes/form.inc b/includes/form.inc
index c0163ca..0309a7e 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -2520,7 +2520,7 @@ function form_process_select($element) {
  */
 function theme_select($variables) {
   $element = $variables['element'];
-  element_set_attributes($element, array('id', 'name', 'size'));
+  element_set_attributes($element, array('id', 'name', 'size', 'multiple'));
   _form_set_class($element, array('form-select'));
 
   return '<select' . drupal_attributes($element['#attributes']) . '>' . form_select_options($element) . '</select>';