Index: modules/forum/forum.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/forum/forum.module,v
retrieving revision 1.424
diff -u -F^f -r1.424 forum.module
--- modules/forum/forum.module	25 Oct 2007 08:24:43 -0000	1.424
+++ modules/forum/forum.module	25 Oct 2007 15:12:05 -0000
@@ -161,6 +161,9 @@ function forum_init() {
  */
 function forum_nodeapi(&$node, $op, $teaser, $page) {
   $vid = variable_get('forum_nav_vocabulary', '');
+  if (empty($vid)) {
+    return;
+  }
   $vocabulary = taxonomy_vocabulary_load($vid);
 
   // Operate only on node types assigned for the forum vocabulary.