The bottom step will NOT fit if the black vertical piece under the top step that it backs up to is used. The step is too deep and sticks out too far, so the slot for the locking tab doesn't line up at all and the tab can't go in. The locking tab then holds the step up so that it can't lie flat. Luckily that black vertical piece is not vital because of the drop-in storage compartment, which closes the gap between the top and bottom steps, although the top step would have better support if it fit and could be used.