anova() method for coxph objects with robust standard errors

Hi all,

what is the reason why the anova() method for coxph objects does not work when robust standard errors have been requested, e.g.,

fit <- coxph(Surv(futime, fustat) ~ resid.ds *rx + ecog.ps, data = ovarian, robust = T)
anova(fit)


any pointers will be much appreciated.

Sincerely,

Mura