Problem Curve Fitting Toolbox using Custom Equation for a Exponential Function

Hello,

I have a problem using a "Custom Equation" for an exponential function. With the use of the model type "Exponential" a*exp(b*x) I get a proper fit.

In order to extend with a constant factor, I used model type "Custom Equation" and entered y=a*exp(b*x).

Here I got an error: "Inf computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients".

All Fit Options like algorithm, start points, etc, are equal (cf. screenshots). It doesn't make a difference if "Center and scale" is activated.

A limitation to the boundaries doesn't help.

Why I can't get a fit with the "Custom Equation"?

Best Regards

Exponential

Custom Equation