Index: modules/update/update.manager.inc
===================================================================
RCS file: /Users/wright/drupal/local_repo/drupal/modules/update/update.manager.inc,v
retrieving revision 1.16
diff -u -p -r1.16 update.manager.inc
--- modules/update/update.manager.inc	30 Jan 2010 07:59:26 -0000	1.16
+++ modules/update/update.manager.inc	31 Jan 2010 17:59:56 -0000
@@ -466,7 +466,7 @@ function update_manager_install_form($fo
   $archiver_info = archiver_get_info();
   foreach ($archiver_info as $info) {
     if (!empty($info['extensions'])) {
-      $extensions += $info['extensions'];
+      $extensions = array_merge($extensions, $info['extensions']);
     }
   }
   $form['help_text'] = array(