[R] formula in nlmer function of lme4 package

[bernardsfr]

Hi, 
I am trying to fit a simple mixed Emax model with lme4 package (R version
2.12.0), with the following code:
 nlmer(Y~exp(EM)*C/(exp(EC)+C)
~(EM+EC|subject),start=c(EM=0.1,EC=-0.2),data=tableau)
and I have the error code in return:

"Erreur dans nlmer(Y ~ exp(EM) * C/(exp(EC) + C) ~ (EM + EC | subject),
start = c(EM = 0.1,  : 
  gradient attribute of evaluated model must be a numeric matrix"

despite the fact that I have already seen such code somewhere, I understand
that instead of 
"Y ~ exp(EM) * C/(exp(EC) + C)"  nlmer is expecting a function that returns
also the gradient matrix in addition to output; it is easy to calculte the
gradient but I have no idea on how this enriched output of this function
should be specified in order to provide the gradient as well; any idea to
help me ?

thanks
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