0 tiles are damaged or missing

[Feycat]

I ran my first load of blister steel just fine and replaced the broken block (yay for the updated visuals!) but now every time I click the coffin it says this. I've gone all around it and can't find anything damaged, but it won't let me load more iron in. Has anyone else had this happen?

[xXx_Ape_xXx]

Try removing the door and replace it? I had something similar happen once, but I can't quite remember what fixed it 

[Volsung]

having this problem as well, still haven't found a solution

[Volsung]

For me at least the "0" part was a lie: I started dismantling my cementation furnace and found a damaged refractory block. While I haven't fully rebuilt the structure, I think it will now work.

[LadyWYT]

I would check the lower layers and the grating to make sure nothing is damaged there; those blocks are harder to see, especially if buried, so it's possible to miss one. If it's winter, make sure all the snow is cleared out from inside the furnace, as it won't function otherwise.