[R] warning message for lmer model with poisson family

[arams]

Hello,

I'm trying to run an lmer model with family poisson but receive the
following
warning message: 

model<-lmer(count~tem+dat+alt+year+tem:dat+tem:alt+tem:year+dat:alt+dat:year+alt:year+(1|id),family=poisson)
Error in objective(.par, ...) : 
  Leading minor of order 2 in downdated X'X is not positive definite

I'm using the newest version of R and recently installed the following
libraries: (lme4, matrix, lattice).
I don't understand what the warning message means or know how to fix it,
please help!
My response variable is a count variable (n=551); 212 with value = 0, and
the rest range between 1-3.
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