Abra o arquivo e procure a linha com o problema:
gksu gedit /etc/apt/sources.list
No meu caso, o arquivo contém isso nas primeiras sete linhas (compare):
# deb cdrom:[Ubuntu 13.04 _Raring Ringtail_ - Release amd64 (20130424)]/ raring main restricted
deb-src http://archive.ubuntu.com/ubuntu raring main restricted #Added by software-properties
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://ubuntu.grn.cat/ubuntu/ raring main restricted
deb-src http://ubuntu.grn.cat/ubuntu/ raring universe main restricted multiverse #Added by software-properties
Verifique se há alguma diferença. Às vezes, devido a uma conexão corrompida, recebi erros devido a linhas incompletas. Boa sorte!