diff -urpN drupal-6.x-dev.org/modules/block/block.module drupal-6.x-dev/modules/block/block.module
--- drupal-6.x-dev.org/modules/block/block.module	2007-05-28 14:08:39.000000000 +0800
+++ drupal-6.x-dev/modules/block/block.module	2007-05-30 11:21:42.000000000 +0800
@@ -295,7 +295,7 @@ function _block_compare($a, $b) {
 function block_admin_display_submit($form, &$form_state, $form_values) {
   foreach ($form_values as $block) {
     $block['status'] = $block['region'] != BLOCK_REGION_NONE;
-    $block['region'] = $block['status'] ? $block['region'] : '';
+    $block['region'] = $block['status'] ? $block['region'] : 'left';
     db_query("UPDATE {blocks} SET status = %d, weight = %d, region = '%s', throttle = %d WHERE module = '%s' AND delta = '%s' AND theme = '%s'", $block['status'], $block['weight'], $block['region'], isset($block['throttle']) ? $block['throttle'] : 0, $block['module'], $block['delta'], $block['theme']);
   }
   drupal_set_message(t('The block settings have been updated.'));