Custim fit doesn't work when adding problem values

Dear All

I am trying to fit some experimental data to a custom fit by using the fit function from the Curve Fitting Toolbox. In my fit I have 3 'problem' parameter (i.e a value i don't fit) which changes for each data point. But when I include them in fit function I get the error:

Error using fittype/testCustomModelEvaluation (line 12) Expression testMem(x,DH,K, Cdi,Vcell,dCl0i) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated: Error in fittype expression ==> testMem(x,DH,K, Cdi,Vcell,dCl0i) ??? Index exceeds matrix dimensions.

My fit function is:

function y=testMem(x,DH,K, Cdi,Vcell,dCl0i)

y=zeros(size(x));

for i=1:length(x)

      y(i)=dCl0i(i)*Vcell(i).*Cdi(1).*DH.*(K./((1+K.*x(i)).^2));

end

end

And I do the fit by the following:

ftype=fittype('testMem(x,DH,K, Cdi,Vcell,dCl0i)','problem', {'Cdi','Vcell','dCl0i'})
f=fit(ClipCelli,data(:,1)*1e-6,ftype,'problem', {Cdert0i,Vtoti,dClipCelli})

All the data and all the 'problem' numbers are 25 x 1 vectors of the type double. And the function IS working if i try ti with reasonable value for my two fit parameters (DH, K)

If I remove all the problem values from my fit is works (in the sense that I don't get any error messages, the fit is totally off).

Any ideas for what is going on here? Any hints what i should check? ALL help is highly appropriated

[Julian Groß-Funk]

Problem variables are only allowed to be one character long. That means only a-z and A-Z are available as problem variables