Error from using rotated_lorentz: why is it concave?

1

I am a newbie to CVX, and am trying to build the following model:

cvx_begin
    obj = 0;
    variable x(A, B);
    variable y(A, B);
    variable z(A, B);
    for i = 1:A
        for j =1:B
             obj = obj + x(i, j)
        end
    end
    minimize(obj)
    subject to
    for i = 1:A
        for j = 1:B
            {x(i, j), y(i, j), C*log(1+C*z(i, j)) / log(2)} == rotated_lorentz(1);
        end
    end
....... (other constraints)
.......
cvx_end

However, I get the error message: Disciplined convex programming error:
Invalid constraint: {concave} == {real affine}.
CVX does show that the constraint of my model is concave, but I have no idea why it is, since many posts seems to do similar things.