[R] contrast in lme

[Jacques Ropers]

Dear R-users

I'm modelling some longitudinal data (1 response variable measured at 6
occasions, 1 baseline, one "treatment" variable) collected in the same
subjects using the following model:
library(nlme)
model.lme <- lme(response ~ V0+ time+ tt + tt:time, random = ~1|subject,
correlation = corSymm(form = ~ 1 | subject), na.action=na.omit,
data=arm.total2)

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
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