Model definition and initial values for model coefficients must be provided.

Hello all,

I have the following error, "Model definition and initial values for model coefficients must be provided.", when I run a non-linear curve fit equation into MATLAB

My script code is as follows:

X = [R75X R75Y];
modelfun = @(b,x)((b(1)*x.^b(2))/(0.25*b(3)-x));
beta0 = [-10^-10 3 30];
mdl=NonLinearModel.fit(X,modelfun,beta0)

R75X and R75Y are both 83 X 1 sets of x and y coordinates respectivelty for each data point.

I have toyed around with the code a little, but it won't budge.

Any ideas?