Pubs Transaction log

I can't backup the transaction log for the pubs database
on any systems I have. I even made some bogus transactions
to have something in the log. Is backing up the Pubs T/log
not allowed? Thanks in advance for your reply.--Denhi Den,
Its because recovery model of pubs database must be simple. change it to
bulk-logged/full and then try taking backup.
- Vishal