Harmony Patch Sealed Class

I'm trying to patch a class that is in a class of its own. The inner class is marked as sealed.

My Code is:

        [HarmonyPatch(typeof(RoomManager.HGEAPFPDOEC))]
        [HarmonyPostfix]
        [HarmonyPatch("_JoinRoomContinuation_b__0")]
        static void _JoinRoomContinuation_b__0()
        {

        }

But after that my process doesn't start anymore.

The inner class -> public sealed class LIIAANCEOMA is declared in -> public class RoomManager

How can I patch that?