Multistrap: Couldn't find package zlib1g*

Hello, emdebbers.
I'm preparing rootfs for my embedded system.
With simple configuration file multistrap works fine:
#sudo multistrap -a armel -d $PWD/RFS -f multistrap.conf
multistrap.conf:
[General]
noauth=true
unpack=true
debootstrap=Grip
aptsources=Grip
[Grip]
#packages=
source=http://www.emdebian.org/grip
suite=squeeze

But there is no tools I need including apt for installing packages
from repos. So I added string "packages=apt" in my simplest config
file.
After that multistrap gave me an error:
E: Couldn't find package zlib1gapt
apt download failed. Exit value: 100
The same trouble goes with any package I want to install in addition
to minimal configuration: "Couldn't find package zlib1ggcc", and so
on..

What I did wrong, or maybe it's a bug? Thank You.

-- 
Best Regards,
Dmitriy Alekseev