Maximum number of function evaluations has been exceeded - increase MaxFunEvals option. Current function value: NaN

While using fminsearch I am getting the following error

Maximum number of function evaluations has been exceeded

- increase MaxFunEvals option.

Current function value: NaN

I am trying to maximise an utility function.

I will provide the code below.

function [utility] = demo(a,initx,b,q,nyears,alpha,beta,discount)

%define output variable x

%b = 0.42;

%initx = 0.0;

%q =2;

%alpha = 0.4

%beta = 0.08;

%discount = 0.98;

%nyears =100;

x = Lakemodel(initx,b,q,a,nyears);

v = zeros(nyears,1);

% Code for Lakemodel you can run it as a different function

%function [x,source,sink] = Lakemodel(initx,b,q,a,nyears)

%define output variable x

%x = zeros(nyears,1);

%x(1) = initx;

%for t = 2 : nyears%

%x(t) = x(t-1) + (a(t-1)) - b*x(t-1) + x(t-1)^q/(x(t-1))^q+1;

%end

%end

for t = 1:nyears

v(t) = discount^t*(alpha*a(t)-beta*x(t)*x(t));

end

utility = sum(v(:,1));

utility = utility*-1;