WHY putting custom Flat texture doesn't work

[fai1025]

so what happen is I try to inport an all black texture for flat

I didn't even make a new lump, I just replace the exist flat of Doom2 with the one I want

the image size is 64x64, yes, I did convert it to Flat

everything set, until this pops out

 

I've been struggle a lot with all the Flat texture

every time I deal with them It always has problem, either doesn't even appear, or something like this. 

I try to not make new lump for Flat, which is where the problem came from usually, but seem like replacing also cause problem

[ViolentBeetle]

I have no idea, can you post the offending wad?

[fai1025]

Never mind, I restart Slade, and it fixed

perhaps this is Slade's problem