I could only reproduce the bug removing /etc/debian_version as well as /etc/apt/sources.list.
Thing is, aptsources.distro from python-apt, has a function, get_distro, which uses lsb_release to get the system codename. lsb_release will give it 'n/a' as the codename and it will not match any of the templates found in /usr/share/python-apt/templates/. I think the installer should install /etc/debian_version properly and this should be reassigned to the debian-installer, if the reporter confirms there was no /etc/debian_version after the installation. Regards, Cascardo.
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