curve fitting: problem using fitoptions

Hey hey,

i try to fit a function. If i use the following code i don't get an error but the fitting is horrible.

g=fittype(sin(pi/2*(Cs*0.5^3*x^4 - 2*Dz*x^2));,...
          'independent','x');
[c2,gof2] = fit(x.',data,g)

Now want to use some other fitting methodes for example:

s = fitoptions('Method','NearestInterpolant');  
g=fittype(sin(pi/2*(Cs*0.5^3*x^4 - 2*Dz*x^2));,...
            'independent','x','option',s);
[c2,gof2] = fit(x.',data,g)

But there i get an error:

??? There is no 'lower' property in the 'interpoptions' class.

Error in ==> fit at 290 lowerbnd = get(options,'lower');

Error in ==> PCTF_fitting at 46 [c2,gof2] = fit(k.',((sin(w))).',g)

Thanks for your help

Phil