cvs problems on Linux (build 666) [ probably my fault ]

I just started to integrate cvs in IDEA. I already use CVS on the command line.

My problem is the following: I've defined a cvs root in the project properties, the same as the one I use on the command line.
:pserver:guest@cvs.tigris.org

When I try an update of a single file in the Editor, I get the following on the console:

/usr/bin/cvs -z 9 update -d -P IssueTypeAttributeEdit.java
Fatal error, aborting.
guest: no such user.

CVS command finished execution


I guess there must be something wrong in my setup.
Any idea?

Jerome