[R] Error messages using LMER

[Shige Song]

Dear All,

After playing with lmer for couple of days, I have to say that I am
amazed! I've been using quite some multilevel/mixed modeling packages,
lme4 is a strong candidate for the overall winner, especially for
multilevel generzlized linear models.

Now go back to my two-level poisson model with cross-classified model.
I've been testing various different model specificatios for the past
couple of days. Here are the models I tried:

1) Two level random intercept model with level-1 covariates only
m1 <- lmer(.D ~ offset(log(.Y)) + (1|provn) +(1|bcohort) + x1 + x2 ,
data, poisson, method="Laplace")

2) Two-level random intercept model with both level-1 and level-2
covariates, but no cross-level interactions:
m2 <- lmer(.D ~ offset(log(.Y)) + (1|provn) +(1|bcohort) + x1 + x2 +
z1 + z2, data, poisson, method="Laplace")

3) Two-level random intercept with cross-level interaction
m3 <- lmer(.D ~ offset(log(.Y)) + (1|provn) +(1|bcohort) + x1 + x2 +
z1 + z2 + x1:z1 + x2:z2, data, poisson, method="Laplace")

Both model 1 and 2 run fine. For model 3, I got error message:
----------------------------------
Error in fn(par, ...) : Unable to invert singular factor of downdated X'X
In addition: Warning messages:
1: optim or nlminb returned message ERROR: ABNORMAL_TERMINATION_IN_LNSRCH 
 in: LMEopt(x = mer, value = cv) 
2: Leading minor of size 1 of downdated X'X is indefinite 
----------------------------------

What is going on here? Any workarounds? Thanks!

Best,
Shige