lme4 and cholmod warning

Dears,

I encoutered the following message, when using lme4 :

Warning messages:
1: In mer_finalize(ans) :
 Cholmod warning 'not positive definite' at
file:../Cholesky/t_cholmod_rowfac.c, line 432
2: In mer_finalize(ans) :
 Cholmod warning 'not positive definite' at
file:../Cholesky/t_cholmod_rowfac.c, line 432
3: In mer_finalize(ans) : singular convergence (7)


I saw the same kind of message on your mailing list, but then I could
not follow up the reply.

My model was:

ga1p<-glmer(A1P~T131006+typop*mr+(1|pop/fam), family= poisson)

The dataset is unbalanced:  there are 16 "pop", with a varying number of
"fam" within pop. In one population, there is only one family.
The problem is related to T131006 (when omitted, glmer works). When I
replace this covariate by another one, I have the same message, except
in one particular case.