base-installer: "failed to determine the codename for the release" (expert mode)

[Leonardo Boiko <leoboiko.mlbox@gmail.com>]

Hi.  I'm trying to netinstall testing on an old laptop via a PLIP
connection, which is possible only in expert mode (I think). 
Everything works fine up to the "install the base system" step, which
fails with the following message: "Debootstrap Error: Failed to
determine the codename for the release".  This happens consistently.

As far as I can tell, base-installer throw this error when it cannot
db_get mirror/codename, but since I don't know debconf I can't fix
that.  The mirror was correctly set up in previous steps.  Any
pointers?
--
Leonardo Boiko