今天我尝试更新包列表，但遇到了奇怪的错误
$ sudo apt-get update
...
Ign http://archive.ubuntu.com saucy-security/universe Translation-en_US
Ign http://archive.ubuntu.com saucy-proposed/main Translation-en_US
Ign http://archive.ubuntu.com saucy-proposed/restricted Translation-en_US
Ign http://archive.ubuntu.com saucy-proposed/universe Translation-en_US
Fetched 496 kB in 38s (13.0 kB/s)
E: Unable to change to (unreachable)// - chdir (2: No such file or directory)
我现在应该改变什么？东西在/etc/apt/sources.list？