[R] a question about using nlme

[Lijuan Wang]

Hi,
I am using Splus to run a multiphase mixed-effects model.
The quations of the models are as below:

gf[ij]=b0[i]+b1[i]*age[ij]+b2[i]*max(0,(age[ij]-tau[i]))^2+e[ij]
b0[i]=b00+e[i0]
b1[i]=b10+e[i1]
b2[i]=b20+e[i2]
tau[i]=tau+e[i3]

i: 1,2,...,100 subjects
j: 1,2,...,6 occasions

The main scripts of Splus is:
simu1<-groupedData(gf~age|id)

simu.nlme<-nlme(gf~(b0 + b1 * age + b2 * 
max(0,(age-tau))^2),data=simu1,fixed=list(b0~1,b1~1,b2~1,tau~1),
+ 
random=list(b0~1,b1~1,b2~1,tau~1),start=c(b0=4,b1=5.32,b2=-5.29,tau=14.8))

But it seems like that there are some errors in it.
Error in .C("mixed_EM",: Singularity in backsolve at level 
2, block 1

I think maybe there is something wrong in my scripts, 
could you help me to find it?
Thanks a lot
Peggy