diff --git a/app/main.py b/app/main.py
index b181561..1b71226 100644
--- a/app/main.py
+++ b/app/main.py
@@ -5,7 +5,7 @@
 # Copyright 2019 (C) Hui Lan <hui.lan@cantab.net>
 # Written permission must be obtained from the author for commercial uses.
 ###########################################################################
-from flask import escape
+from flask import escape, abort
 from Login import *
 from Article import *
 import Yaml