Query: May one sleep underneath the Dofen Akumah?

Answer (R. Yitschak ben Elyashiv): Liquid mud can be added to a Mikveh to complete the minimum volume needed for the Mikveh to be valid, but one cannot actually immerse in the mud. Here also, the invalid Schach can be used to validate the Sukah but one cannot sleep underneath it.

Chizkiyah: The reason that the Mishnah allows invalid Schach between the walls and the Schach is because we view the invalid Schach as part of the walls, but in the middle of a Sukah, invalid Schach invalidates the Sukah if it is four Tefachim wide.

Beraisa (R. Hoshiya) as well as R. Ba/ R. Chiya quoting R. Yochanan and R. Zeira/ R. Yasa quoting Kahana: (Like Chizkiyah) We view the invalid Schach as part of the walls.

R. Yona to R. Yosi: Why can we not say that the reasoning of the Mishnah is that invalid Schach invalidates when it is four Amos, both on the side and in the middle?