Magnetic field due to an infinite slab, where its two halves have current in opposite direction

Say we have a uniform current density J = J zˆ exists in a slab between the infinite planes x = −b and x = +b, but the left half -b to 0 has its current coming out of the page while the right half is going into the page. We are trying to find the B field everywhere so both inside and outside the slab. Now if we simply applied ampere’s law with x=0 being the centre of our amperian loop (a rectangle) we would get I=0 so B=0, which seems incorrect? What would be the right way to go about this problem? Should we treat each half of the slab separately first and then add their B field via superposition principle? How do we tell when we are abusing planar symmetry incorrectly?