Idk why Cell contents reference from a non-cell array object in my file

How do i fix: Cell contents reference from a non-cell array object.

Error in RKSystems (line 27) k(1, j) = h*f{j}(t, w);

 %
function RKSystems(N)
    %N = iterations
    %m is the vector size
    m=4;
    h = 10^-2;        %step size
    t = 0;            %time start 
    v = 1000;         %initial velocity
    theta = pi/4;     %angle of launch

    %f is the ODE vector 
    %What do i need to put for 'y1' part? w(3)?
    f= @(t,x) [ x(2); -.02*(x(2)/(1+x(3)/2000)); x(4); -.05*x(4)/(1+x(3)/2000) - 9.8;];

    w = zeros(1, m);
    w(1,:) = [0, v*cos(theta), 0, v*sin(theta);]; %initial condition row vector

    fprintf('t = %.2f;', t);
    for i = 1:m
        fprintf(' w(%d) = %.10f;', i, w(i));
    end
    fprintf('\n');

    k = zeros(4, m);
    for i = 1:N
        for j = 1:m
           k(1, j) = h*f{j}(t, w);
        end

        for j = 1:m
            k(2, j) = h*f{j}(t + h/2, w + (1/2)*k(1, :));
        end

        for j = 1:m
            k(3, j) = h*f{j}(t + h/2, w + (1/2)*k(2, :));
        end

        for j = 1:m
            k(4, j) = h*f{j}(t + h, w + k(3, :));
        end

        for j = 1:m
            w(j) = w(j) + (k(1, j) + 2*k(2, j) + 2*k(3, j) + k(4, j))/6;
        end

        t = a + i*h;

        fprintf('t = %.2f;', t);
        for k = 1:m
            fprintf(' w(%d) = %.10f;', k, w(k));
        end
        fprintf('\n');

    end 
end