Index: modules/book/book.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/book/book.module,v
retrieving revision 1.406
diff -U3 -r1.406 book.module
--- modules/book/book.module	9 Jan 2007 08:31:22 -0000	1.406
+++ modules/book/book.module	31 Jan 2007 21:13:37 -0000
@@ -277,7 +277,7 @@
   );

   $form['nid'] = array('#type' => 'value', '#value' => $nid);
-  if ($node->parent) {
+  if (isset($node->parent)) {
     $form['update'] = array('#type' => 'submit',
       '#value' => t('Update book outline'),
     );