[R-sig-ME] error with glmer

[paola berchialla]

Dear all, I have some troubles in using lme4 package, glmer function.
Here it is the code I am trying to ran unsuccessfully:

mod.glmer=glmer(Cmis~X_Igruppo_2+X_Igruppo_3+X_Igruppo_4+(1|mon),data=p4,family=Gamma(link='log'))
summary(mod.glmer)

The problem is I get the following error message: Error in 
asMethod(object) : matrix is not symmetric [1,2]
Could someone please explain this error to me? I am using R version 2.7.2