[R] precision matrix for polynomial growth curves

[Robert Cuffe]

What does the warning message 
"1: Singular precision matrix in level -1, block 1" mean?

I get this warning 50+ times when I try to fit the following
model

lme( response ~ covariateA + poly(covariateB,3), ~poly(covariateB,3)|group )

It's not a small dataset - a set of up to 20 blood pressure
readings on just over 2000 people, and I don't get the error
message when I try to fit fewer random effects (although I'm
not sure whether fitting random effects poly(B,2) with fixed
effects poly(B,3) is generally the same as fitting random
effects B+B^2 with fixed effects B+B^2+B^3).

Sometimes I don't get the error message when I use subsets of
the data, but that isn't guaranteed to be the case.

If anybody could give me pointers to references for or
could answer either question themselves (1. why the error
message/what's causing it and 2. fitting random polynomials
using poly()) I would be very grateful.