[R] impenetrable warning

[Simon.Bond]

Dear R-help,

Can anyone explain the meaning of  the warning,

Singular precision matrix in level -1, block 1

? Or how to track down where it comes from?

More precisely, using the nlme package, I'm issued with the warning

itt2 <- lme(lrna~rx.nrti+lbrna, random=~1|patid,
cor=corExp(form=~days|patid,nugget=T), weights=varPower(
form=~lbrna),data=rna3)
Warning messages:
1: Singular precision matrix in level -1, block 1
2: Singular precision matrix in level -1, block 1

the output is:

Linear mixed-effects model fit by REML
  Data: rna3
  Log-restricted-likelihood: -4990.142
  Fixed: lrna ~ rx.nrti + lbrna
   (Intercept) rx.nrtiZDV+3TC rx.nrtiZDV+ABC          lbrna
     2.6597552      0.8589514      0.4259504      0.2929222

Random effects:
 Formula: ~1 | patid
        (Intercept)  Residual
StdDev:    1.251113 0.2105329

Correlation Structure: Exponential spatial correlation
 Formula: ~days | patid
 Parameter estimate(s):
      range      nugget
204.6422150   0.3349336
Variance function:
 Structure: Power of variance covariate
 Formula: ~lbrna
 Parameter estimates:
    power
0.9733358
Number of Observations: 2390
Number of Groups: 124



Thanks,

Simon Bond.