Index: modules/user/user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.1039
diff -u -p -r1.1039 user.module
--- modules/user/user.module	5 Sep 2009 05:45:45 -0000	1.1039
+++ modules/user/user.module	8 Sep 2009 08:20:21 -0000
@@ -2910,12 +2910,19 @@ function user_block_user_action(&$object
     global $user;
     $uid = $user->uid;
   }
-  db_update('users')
-    ->fields(array('status' => 0))
-    ->condition('uid', $uid)
-    ->execute();
-  drupal_session_destroy_uid($uid);
-  watchdog('action', 'Blocked user %name.', array('%name' => $user->name));
+  
+  if ($uid) {
+    $account = user_load($uid);
+    db_update('users')
+      ->fields(array('status' => 0))
+      ->condition('uid', $uid)
+      ->execute();
+    drupal_session_destroy_uid($uid);
+    watchdog('action', 'Blocked user %name.', array('%name' => $account->name));
+  }
+  else {
+    watchdog('action', 'Attempted to block a user but no UID was supplied.');
+  }
 }
 
 /**