[R-sig-ME] Error message from nlmer

[Kari Ruohonen]

I apologise that I re-post this question but I just learnt about this
specific mixed model list, and since I got no replies from r-help I
thought there may be people able to help on this list that are not
reading the main r-help.

Once more my sincere apologies if someone feels this is misuse of the
list. I would appreciate very much any hints to solve the problem
described below since I am totally puzzled.

-------- Forwarded Message --------
From: Kari Ruohonen <kari.ruohonen at utu.fi>
To: r-help at r-project.org
Subject: Error message from nlmer
Date: Thu, 25 Oct 2007 15:31:46 +0300

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