R help - Oct 2007 - Error message from nlmer

Hi,

I have R 2.6.0 with updated lme4 and Matrix packages, and I am trying to
fit a nonlinear multilevel model. I get the following error message:

Error in nlmer(f ~ grModel(x, w, Tmin, Tmax, Topt, kopt, m) ~ kopt |
flat,  : 
  gradient attribute of evaluated model must be a numeric matrix

and I wonder what this may indicate.

The nonlinear model I try to fit is as follows:
> grModel
function(x,w,Tmin,Tmax,Topt,kopt,m)((x-Tmin)*(x-Tmax))/((x-Tmin)*(x-Tmax)-(x-Topt)^2)*kopt*w^m

and my call of nlmer is as follows:

nlmr1<-nlmer(feed~grModel(temp,MW,Tmin,Tmax,Topt,kopt,m)~kopt|
flat,data=egi,start=c(Tmin=0.2,Tmax=21.1,Topt=14.6,kopt=2.5,m=.51))

'flat' is a factor with 11 levels.

My start values are from nls that fits the model with no errors. The
call I have used is as follows:

nls1<-nls(feed~grModel(temp,MW,Tmin,Tmax,Topt,kopt,m),data=egi,start=c(Tmin=1,Tmax=20,Topt=13,kopt=3,m=.7))

The examples given on the help page of nlmer run with no errors.

I would appreciate any hints and help to track the problem indicated by
the error message.

Thanks,

Kari