R help - Oct 2005 - schoenfeld residuals in the cox.zph function

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I work with a clinical dataset including about 27,000 patients and I wanted to
do some model diagnostics on cox regression models with frailty effects (for
every patient). But in this case the cox.zph function did not work. I always got
the following error message:

Fehler in residuals.coxph(fit, "schoenfeld") :
        NA/NaN/Inf in externem Funktionsaufruf (arg 5)

Does anyone know what's wrong? I did the same without using frailty effects
and everything was o.k. What could I do?