Query:problem with the corStruct Classes.

Dear all,

In Pinheiro and bates'book, we fit an ARMA(1,1) model with:

fm5Ovar.lme<-update(fm1Ovar.lme,corr=corARMA(p=1,q=1)

But the result is:
Error in "coef<-.corARMA"(*tmp*, value = c(62.3428455940029, 
95.0395441938099,  : 
        Coefficient matrix not invertible

There is also problems when we fit an exponential spatial correlation 
model for the within-group error:
fm3BW.lme<-update(fm2BW.lme,corr=corExp(form=~Time))
We obtain:
Error in recalc.corSpatial(object[[i]], conLin) : 
        NA/NaN/Inf in foreign function call (arg 1)

I would be grateful if anyone could help me.

Cordially,

Martin Ludovic.