problem computing dfbeta by residuals.coxph in model with a cluster term

["Hunsicker, Lawrence" <lawrence-hunsicker@uiowa.edu>]

Hi, all:

I'm back to the trough with a very specific problem.  I should like to
examine the dfbeta residuals of a multiple-event cox model that includes a
cluster term.  One of the covariates in the model is "not.first".  The model
specification is:

        coxph(formula = Surv(start, stop, event) ~ C(trt, plac.ref) +
not.first + cluster(pid), data = chfc.wei)

When I run the following:

        dfb<-residuals.coxph(chfc.fit1,type='dfbeta')

I get the following error message:

        Problem in model.frame.coxph(fit): Length of not.first (variable 3)
is 1 
        != length of others (2030) 

If I delete the 'not.first' covariate, the function works fine.  I should
like to include the not.first, as it turns out to be an important
explanatory variable.  Am I doing something wrong, or is there a glitch in
the program?

Larry Hunsicker