diff --git a/src/gl/data/gl_setup.cpp b/src/gl/data/gl_setup.cpp
index a6a0141f3..fb9c2252f 100644
--- a/src/gl/data/gl_setup.cpp
+++ b/src/gl/data/gl_setup.cpp
@@ -348,7 +348,7 @@ static void PrepareTransparentDoors(sector_t * sector)
 
 				int side = sector->lines[i]->sidedef[0]->sector == sec;
 
-				if (sector->GetPlaneTexZ(sector_t::floor)!=sec->GetPlaneTexZ(sector_t::floor)+1.) 
+				if (sector->GetPlaneTexZ(sector_t::floor)!=sec->GetPlaneTexZ(sector_t::floor)+1. || sec->floorplane.isSlope())
 				{
 					sector->transdoor=false;
 					return;