diff -u b/core/modules/forum/config/forum.settings.yml b/core/modules/forum/config/forum.settings.yml
--- b/core/modules/forum/config/forum.settings.yml
+++ b/core/modules/forum/config/forum.settings.yml
@@ -11 +11 @@
-vocabulary: '0'
+vocabulary: forums
diff -u b/core/modules/forum/forum.install b/core/modules/forum/forum.install
--- b/core/modules/forum/forum.install
+++ b/core/modules/forum/forum.install
@@ -248,10 +248,9 @@
 }
 
 /**
- * Update forum module to use the configuration system.
+ * Move Forum variables to the configuration system.
  */
 function forum_update_8000() {
-  config_install_default_config('forum.settings');
   update_variables_to_config('forum.settings', array(
     'forum_hot_topic' => 'topics.hot_threshold',
     'forum_per_page' => 'topics.page_limit',