MATLAB odd 'Too many input arguments' error

For a project, I'm trying to use matlab to call a function in another .m file. However, it says 'Not enough input arguments', even though I do pass what I am fairly certain is enough input arguments.

In eval_square.m:

function f = eval_square(x)

%   fitness function of the magic square
%
%   Parameters
%   ----------
%       x : array, the solution vector that represents a magic square.
%           By default, the solution vector is converted to a magic square
%           columnwisely.
%   Output
%   ----------
%       f : double, the error value of the input solution vector.
%           the mean squared error (MSE) of all each row, column and
%           diagonal sum to the magic constant is computed
%

n = sqrt(length(x));

%More stuff, but error occurs at this line.

in MYNAME_sa.m:

function [xopt, fopt] = MYNAME_sa(dim, eval_budget, fitness_func)

%Stuff

if dim == 2
    len = 12^2;    % length of the solution vector, shoud be 12^2 
                 % when dim == 2
elseif dim == 3
    len = 7^3;    % length of the solution vector, shoud be 7^3 when 
                 % dim == 3
end

%Stuff

s = randperm(len)         
f = fitness_func(s)

%More stuff.

It's supposed to evaluate the random permutation of length 12^2 as a magic square, to see how close it is to optimum (i.e. how close it is to being an actual magic square) and in theory the same for a magic cube (eval_cube), but the same error occurs.

The error in question:

>> MYNAME_sa(2, 10000, eval_square)
Error using eval_square (line 18) 
Not enough input arguments.

Note that line 18 is n = sqrt(length(x));

It doesn't matter if I hardcode eval_square into the function - it seems to understand that I want to call eval_square just fine, but it just doesn't pass s or something? And I don't understand why. I tried hardcoding n to 12 as well, but that doesn't work either, the error pops up when I'm trying to actually use x then. Changing fitness_func to @fitness_func also changes nothing. So my question is, why does this happen and how do I fix it?