lme: random effects of a quadratic term

Hello,

I am estimating the following model:

so2.lme<-lme(so2~1+I(alcadakm^2)+dia,data=subjectes2,na.action=na.omit)

And when I try to plot the random effects of the quadratic term with
respect to a covariate (mam) I get an error:

so2.lmeRE<-ranef(so2.lme,augFrame=T)
plot(so2.lmeRE,form=I(alcadakm^2)~mam)

Error in plot.ranef.lme(so2.lmeRE, form = I(alcadakm^2) ~ mam ) : 
	Only single effects allowed in left side of form.

Any suggestion?

Thanks!

Montse Rue