Index: modules/block/block.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.admin.inc,v
retrieving revision 1.89
diff -u -p -r1.89 block.admin.inc
--- modules/block/block.admin.inc	4 Oct 2010 18:00:45 -0000	1.89
+++ modules/block/block.admin.inc	9 Oct 2010 21:37:15 -0000
@@ -148,8 +148,10 @@ function block_admin_display_form($form,
      );
     }
   }
-  // Do not allow disabling the main system content block.
-  $form['blocks']['system_main']['region']['#required'] = TRUE;
+  // Do not allow disabling the main system content block when it is present.
+  if (isset($form['blocks']['system_main']['region'])) {
+    $form['blocks']['system_main']['region']['#required'] = TRUE;
+  }
 
   $form['actions'] = array(
     '#tree' => FALSE,