Custom Chest with a Custom Loot Table

So, I made a tombstone block, and if you go through the Tombstone, it's supposed to have very good loot. I made a custom GUI for it (which if it just had the default chest option I would have used that, but as far as I know it does not) and maybe I did the loot chest wrong, but when I tried to Data merge the Loot Table with the Tombstone, absolutely NOTHING is there when I open the GUI. Help? Thanks.