diff --git a/user_mailman_register.pages.inc b/user_mailman_register.pages.inc
index d2c040a..816fafd 100644
--- a/user_mailman_register.pages.inc
+++ b/user_mailman_register.pages.inc
@@ -141,7 +141,7 @@ function user_mailman_register_form($form, &$form_state, $account = NULL, $show_
 
   if (!$lists_available) {
     $form['user_mailman_register']['lists']['#description'] = t('No mailing lists are available.');
-    $form['user_mailman_register']['lists']['#attributes']['class'] = 'umr_list_nolists';
+    $form['user_mailman_register']['lists']['#attributes']['class'][] = 'umr_list_nolists';
   }
   elseif ($show_submit) {
     if (!$account || $account->uid === 0) {