[R-sig-ME] Contrasts with nlme

[Jacques Ropers]

Dear All,

I'm modelling some longitudinal data (1 continuous
response variable measured at 6 times (covariate time taken as factor), 
1 continuous baseline, one "treatment" variable also taken as a factor) 
collected in the same subjects using the following model:


model.lme<- lme(response ~ V0+ time + tt + tt:time, random = 
~1|subject,correlation = corSymm(form = ~ 1 | subject), 
na.action=na.omit, data=total)

Then I would lke to estimate the effect of "treatment" at time = 6 using 
the package "contrast"

library(contrast)
contrast(model.lme ,
a = list(tt="1",time="6"),
b = list(tt="0",time="6")
)

But I get the following error message
"Erreur dans gendata.default(fit = list(modelStruct = list(reStruct = 
list(:   not enough factors"

I'm obviously doing something wrong.

Thanks for your help.

Jacques