Fit harmonics code brings back an error

My code is :

nharm = 1 ;
L = 365.25;
cutoff = 0;
[amp,phase,frac,offset,yy]=fit_harmonics(SSTsummer,t,nharm,L,cutoff);
pred_summer= amp*cos(((2*pi*t)/L)+phase)+offset;

If I run this it brings back and error with this line:

[amp,phase,frac,offset,yy]=fit_harmonics(SSTsummer,t,nharm,L,cutoff);
but doesn't say what the error is and can't find the error.