aptitude loses the lists after "update"ing while offline

[Stefan Monnier <monnier@iro.umontreal.ca>]

If I do

   aptitude update

while my proxy is down (or when I'm offline), the previous package lists
are apparently gone, so after that, any aptitude command will tell me
things like:

   # aptitude install foo
   E: The value 'testing' is invalid for APT::Default-Release as such a release is not available in the sources
   E: The value 'testing' is invalid for APT::Default-Release as such a release is not available in the sources
   #

Of course, the obvious answer would be "don't do that", but the "update"
is also done automatically by unattended-updates, so I don't have complete
control over it.

Why does it do that?  How can I prevent it?


        Stefan