passing an absolute path for sources_list to cache.update does not work

Using python-apt 1.1.0~beta1build1.

Calling cache like such:

cache = apt.Cache(memonly=True, rootdir='/home/seitz/tmp/apt_root')
cache.update(sources_list='/home/seitz/tmp/apt_root/etc/apt/sources-zesty.list')

Would produce an empty cache :( If I copy the contents of sources-zesty.list to sources.list in that same directory and pass that path as an argument, it functions properly. I am looping over several Ubuntu releases as including them all in one sources.list caused conflicts.

Please advise.