It seems that when trying to install any package which requires the creation of a system User and Group, the installer fails with the error: install: invalid user 'username'

When it happened the first time, I assumed it was related to the package, or it's installation script. But now I am attempting a different package, and alas, the same issue arises!

note: /etc/passwd and /etc/group is not chattr -i

Here is the example output from an attempted installation (of varnish):

Selecting previously unselected package varnish.
Unpacking varnish (from .../varnish_3.0.2-1ubuntu0.1_amd64.deb) ...
Processing triggers for man-db ...
Setting up libvarnishapi1 (3.0.2-1ubuntu0.1) ...
Setting up varnish (3.0.2-1ubuntu0.1) ...
install: invalid user `varnish'
dpkg: error processing varnish (--configure):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for libc-bin ...
ldconfig deferred processing now taking place
Errors were encountered while processing:
 varnish

note: running ubuntu 12.04

For now, I have resolved the issue by manually creating the users, and running apt-get install -f, but this does not resolve the actual issue. Any ideas?