[R] lmer - error asMethod(object) : matrix is not symmetric

[Luisa Carvalheiro]

Dear R-help,

I  am having problems using package lme4.

I am trying to analyse the effect of a continuous variable (Dist_NV)
on a count data response variable (SR_SUN) using Poisson error
distribution. However, when I run the model:

summary(lmer((SR_SUN)~Dist_NV + (1|factor(Farm_code)) ,
family=poisson, REML=FALSE))

1 error message and 1 warning message show up:

in asMethod(object) : matrix is not symmetric [1,2]
In addition: Warning message:
In mer_finalize(ans) : singular convergence (7)

A model including  Dist_NV together with other variables runs with no problems.
What am I doing wrong?

Thank you,

Luisa