curve fitting toolbox error

I am trying to fit data to this equation b*(coth(x/a)-a/x) using the curve fitting toolbox. I get this error NaN computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients. Here is the screenshot: https://www.dropbox.com/s/2bgx89frrx2pjbo/b_h.jpg

Answer 1

[Matt J]

You must prevent the search from including a=0. Otherwise, you will have division by 0 operations x/a.