diff --git a/src/gl/scene/gl_sky.cpp b/src/gl/scene/gl_sky.cpp
index 19d1c9127..7fe92b890 100644
--- a/src/gl/scene/gl_sky.cpp
+++ b/src/gl/scene/gl_sky.cpp
@@ -289,7 +289,7 @@ void GLWall::SkyTop(seg_t * seg,sector_t * fs,sector_t * bs,vertex_t * v1,vertex
 		if (frontreflect > 0)
 		{
 			float backreflect = bs->GetReflect(sector_t::ceiling);
-			if (backreflect > 0 && bs->ceilingplane.fD() == fs->ceilingplane.fD())
+			if (backreflect > 0 && bs->ceilingplane.fD() == fs->ceilingplane.fD() && !bs->isClosed())
 			{
 				// Don't add intra-portal line to the portal.
 				return;
@@ -368,7 +368,7 @@ void GLWall::SkyBottom(seg_t * seg,sector_t * fs,sector_t * bs,vertex_t * v1,ver
 		if (frontreflect > 0)
 		{
 			float backreflect = bs->GetReflect(sector_t::floor);
-			if (backreflect > 0 && bs->floorplane.fD() == fs->floorplane.fD())
+			if (backreflect > 0 && bs->floorplane.fD() == fs->floorplane.fD() && !bs->isClosed())
 			{
 				// Don't add intra-portal line to the portal.
 				return;
diff --git a/src/r_defs.h b/src/r_defs.h
index abf43fdf1..2c1a7cfcb 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -955,6 +955,11 @@ public:
 		return LowestFloorAt(a->Pos(), resultsec);
 	}
 
+	bool isClosed() const
+	{
+		return floorplane.Normal() == -ceilingplane.Normal() && floorplane.D == -ceilingplane.D;
+	}
+
 	double NextHighestCeilingAt(double x, double y, double bottomz, double topz, int flags = 0, sector_t **resultsec = NULL, F3DFloor **resultffloor = NULL);
 	double NextLowestFloorAt(double x, double y, double z, int flags = 0, double steph = 0, sector_t **resultsec = NULL, F3DFloor **resultffloor = NULL);