[R-sig-ME] Mixed model and Singularity Error message

[Antoine PACCARD]

Dear Mix modelers,

I know that this topic has been discussed in the past but since I  
cannot find any clear answer yet I am sending this message. I am  
runing a simple mix model as:

m1 <- lme(fixed=trich~habitat*site*subsite*treatment, random=~1|fam/ 
id,data=data,na.action=na.exclude)

and I constantly find this error message:"singularity in backsolve at  
level 0 block 1". I am not sure how to solve the problem. I noticed  
that it arises only when the variable "subsite" is added.
For clarification, all of my fixed effects are factors (excluding  
"trich")

Could anyone help me with that?

Many thanks,
Antoine