aptitude: modifying sources.list

[Jonathan Kaye <jonathan.kaye@univie.ac.at>]

Hi Debianers,
I'm running sarge 2.4.27-1-386 and normally use aptitude (ver.0.2.15.8) 
for handling my packages. I noticed that if I add a new source to 
sources.list and then run "aptitude update", I get the hated
"W: Couldn't stat source package list.." error message.
If I first run apt-get (apt 0.5.27) then everything works perfectly. If 
I then run aptitude update (probably vacuously since I guess apt-get 
update has already done what needs to be done), no more error messages. 
I scoured the man pages, help files, and googled around quite a bit and 
only found one thread that dealt with this question. The thread seemed 
to presuppose that everyone knows you do apt-get update and NOT aptitude 
update after modifying you sources.list file.

First, is this true? or have I misconfigured something?
Second, if true, is it documented somewhere?

Thanks for the help.
Jonathan