Plot of the function with three independent variables in MATLAB

Question

I have a code in MATLAB that I want to plot, but I am facing an error. Does anyone know how to fix this code?

a = 1;
E_0 = 0;

k_x = linspace(-pi/a, pi/a, 100);
k_y = linspace(-pi/a, pi/a, 100);
k_z = linspace(-pi/a, pi/a, 100);
E_k = zeros(length(k_x),length(k_y),length(k_z));
for i = 1:length(k_x)
  for j = 1:length(k_y)
    for k=1:length(k_z)
    E_k(i,j,k) = E_0 + 8*(cos(k_x(i)*a/2)*cos(k_y(j)*a/2)*cos(k_z(k)*a/ 2));
    end
  end
end

I'm looking to get energy levels.