Invalid operation: {positive convex} - {positive convex}

Nonconvex
1

I want to minimize a vector x1 and make sure the elements x1(1)^2+x1(2)^2=x1(3)^2+x1(4)^2

cvx_begin quiet
    variable x1(M*N)
    minimize(norm(x1,2))
    subject to
        x1(1)^2+x1(2)^2-x1(3)^2-x1(4)^2 == 0      
cvx_end

and get an error
Disciplined convex programming error:
Invalid operation: {positive convex} - {positive convex}

2

That constraint is non-convex. Why isn't CVX accepting my model? READ THIS FIRST!