How do i make bustable blocks?

I am making a wad currently, and, i can't do a bustable block, i click on the wall i want bustable,and use the lindef action: 3D-Floor Over Floor: Bustable block
But when i test my map, sonic dies right at start.
so i was wondering, how do i make a bustable block?

You're doing it wrong. You must make a control sector outside the level with the floor and ceiling height you want, along with the textures. You put a linedef on that sector, giving it a unique tag number. Then, you go to the sector that you want to make into a bustable block and you assign it the same sector tag as the linedef.

All your questions will be answered, if you take the time to go on srb2.org and go under editing for levels. Then click on Doombuilder tutorial and it will show you all the basic's. You need to make a control sector for that area. Make a separate square away from the level and put the same sector tag of the desired area. Under sector events, choose the bustable block option on that linedef of the control sector. If done right the sector that is bustable should glow, a yellowish color.