Unreal Engine 4.14 Inverting UV's in postprocess material not working?

Hello, i’m trying to flip on the x-axis everything that the player’s camera is capturing. I’ve been using a postprocess material, getting the tex coords and modifying the the U coordinate, so far if i use a positive value it behaves as expected but if i go below 0, i get a black screen. Recently i found out the TexCoord node has a mirror coord checkbox that i tried to use in order to get what i wanted but i still get a black screen when activated.

Am i doing something wrong?