R help - Jul 2007 - Cox model with interaction

Dear R users,
I am trying to fit a Cox model such as:

Surv(time,event) ~ X1+X1:X2

or 
Surv(time,event) ~ X1*X2 -X2, 

This code is working with coxph but not with cph (nor with psm), and the error
message is:

Error in if (!length(fname) || !any(fname == zname)) { : 
        missing value where TRUE/FALSE is required

Any idea about the cause of the problem and how to manage it within the Design
library?

Thanks a lot for your help.

Rosalba

Dr. Rosalba Miceli
Unit? di Statistica Medica e Biometria
Fondazione IRCCS Istituto Nazionale Tumori
Via G. Venezian, 1
20133 Milan (Italy)
Tel:????????????? +39 2 23903198 / 23902456
Fax:???????????? +39 2 23902095
E-Mail:??????? rosalba.miceli at istitutotumori.mi.it
<www.istitutotumori.mi.it/biometria>