Cftool giving wildly different fits for a Fourier series and the exact same equation in custom fit

My data consists of y values with exponents of e-21 to e-23, which for some reason the custom equation fit can't handle. Could this be a precision error?

The reason I can't use the Fourier series is because I need the custom equation to be a sum of cos functions, and regardless of the custom equation, the lowest y bound I seem to be able to get is e-9.