[R] predict.glmmPQL Problem

[rsubcriber]

Dear all,

for a cross-validation I have to use predict.glmmPQL() , where the 
formula of
the corresponding glmmPQL call is not given explicitly, but constructed 
using as.formula.
However, this does not work as expected:

x1<-rnorm(100); x2<-rbinom(100,3,0.5); y<-rpois(100,2)
mydata<-data.frame(x1,x2,y)

library(MASS)
# works as expected
model1<-glmmPQL(y~x1, ~1 | factor(x2), family=poisson, data=mydata)
predict(model1, newdata=mydata, type="response")

f<-as.formula(paste("y", "~","x1"))
# predict does not work:
# Error in mCall[["fixed"]][-2] : object is not subsettable
model2<-glmmPQL(f, ~1 | factor(x2), family=poisson, data=mydata)
predict(model2, newdata=mydata, type="response")

Has anyone an idea what goes wrong?
I am using R 2.0.1 under Windows 2000.

Thanks,
Anneke