Curve fitting tool problem- upper and lower bounds

I have graphed this surface[ z= -b*log(m*x) + c*y] in the normal matlab setting and I am not trying to see if it fits my data.

But is keeps saying "Ignoring NaNs in data. Inf computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients. "

Here is a screen shot of my bounds.

I am sorry but every time I enter my own equation in here it says this, can you explain how to avoid this, what is happening? it does it for even very simple equations.