Error using cfit/subsref when trying to fit a smoothingspline

According to documentation, if a fittype is specified, the output becomes [curve,gof] = fit(x,y,fttype)

My code is shown as follows:

A = importdata('exp65_C1.SS');
[x,y] = prepareCurveData(A(:,1),A(:,2));
[fit_o,~] = fit(x,y,'smoothingspline');

I get the error:

Error using cfit/subsref
Too many output arguments.

When I execute the fit line. I checked that x and y are both single column vectors of size 12776x1, and both have the same dimensions. The data goes from 0 to 0.83 for x, and 0 to 726 for y.

When I remove the ~ since it said I have too many output arguments, it gives this error instead:

Error using cfit/subsref>iParenthesesReference (line 36)
Too many inputs to CFIT/SUBSREF.

Error in cfit/subsref (line 15)
        out = iParenthesesReference( obj, currsubs );

Which makes sense since fttype requires more than one output. Same error if I don't specify an output at all.

What is going on?