nlme plot residuals per group

dear list:

I used the nlme library according to the great Pinheiro/Bates book, on
R2.3, WinXp

Lac.lme is an lme object with unbalanced data, group is a factor
variable with three levels, when I tried to plot the residuals by
group I got this error msg:

plot(Lac.lme,resid(.,type='p')~fitted(.)|group)

Error in limits.and.aspect(prepanel.default.xyplot, prepanel = prepanel,  :
        need at least one panel

Also When I try to use the auPred() function I get the follwoing error msg:

plot(augPred(Lac.lme))

Error in tapply(as.character(object[[nm]]), groups, FUN[[dClass]]) :
        arguments must have same length

Any suggestions?

Thanks


--
Osman O. Al-Radi, MD, MSc, FRCSC
Fellow, Cardiovascular Surgery
The Hospital for Sick Children
University of Toronto, Canada