lb build 404 error for squeeze/updates/main sub directory

[Chris <bbshliu@gmail.com>]

my auto/config:

#!/bin/sh

lb config noauto \
    --mirror-bootstrap http://mirror.dlut.edu.cn/debian \
    --mirror-chroot-security http://mirror.dlut.edu.cn/debian \
    --mirror-binary http://mirror.dlut.edu.cn/debian \
    --architecture i386 \
    --linux-flavours 686 \
    --packages-lists "standard" \
    --packages "iceweasel emacs" \
    "${@}"

when I run lb build, I got this error:
Get:4 http://mirror.dlut.edu.cn squeeze-updates Release [41.5 kB]
Err http://mirror.dlut.edu.cn squeeze/updates/main i386 Packages
  404  Not Found [IP: 2001:da8:a800::164 80]
Get:5 http://mirror.dlut.edu.cn squeeze/main i386 Packages [8660 kB]


I tried several mirrors. This error still exists. Could you please give 
me some hint to solve it?