fittype with user defined fit-function

I have data where I create a fit from:

    fit_curve=fit(x,y,'smoothingspline'); 
    % x, y long vectors

This fit I use to define a fittype:

   ft=fittype(@(a,b,x) a+fit_curve(x+b));

this fittype I use in another fitfunction

    fit_2=fit(k,l,ft, 'StartPoint', [k(2) l(2));
    % k, l vectors with 3 elements

this makes a beautiful fit. But if I change the fittype to

   ft=fittype(@(a,b,x) a*fit_curve(x*b));

then the fit_2 is totally wrong.

I tried already .* or only replace one of the + by *. How to solve the problem? Let me know, if you need more information or the plots of the curves.