diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index ec17449..e145856 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -1306,6 +1306,6 @@ function _forum_update_forum_index($nid) {
  *
  * @ingroup themeable
  */
-function theme_forum_form($variables) {
+function theme_forum_form(array $variables) {
   return drupal_render_children($variables['form']);
 }