diff --git a/src/maploader/postprocessor.cpp b/src/maploader/postprocessor.cpp
index 8428c32be1..74dc576ead 100644
--- a/src/maploader/postprocessor.cpp
+++ b/src/maploader/postprocessor.cpp
@@ -459,23 +459,26 @@ DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetLineVertexes)
 	return 0;
 }
 
-DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetLineSideRefs)
+DEFINE_ACTION_FUNCTION(DLevelPostProcessor, FlipLineSideRefs)
 {
 	PARAM_SELF_PROLOGUE(DLevelPostProcessor);
 	PARAM_UINT(lineidx);
-	PARAM_UINT(sideidx1);
-	PARAM_UINT(sideidx2);
 
-	if (lineidx < self->Level->lines.Size() &&
-		sideidx1 < self->Level->sides.Size() &&
-		sideidx2 < self->Level->sides.Size())
+	if (lineidx < self->Level->lines.Size())
 	{
 		line_t *line = &self->Level->lines[lineidx];
-		side_t *side1 = &self->Level->sides[sideidx1];
-		side_t *side2 = &self->Level->sides[sideidx2];
+		side_t *side1 = line->sidedef[1];
+		side_t *side2 = line->sidedef[0];
 
-		line->sidedef[0] = side1;
-		line->sidedef[1] = side2;
+		if (!!side1 && !!side2) // don't flip single-sided lines
+		{
+			sector_t *frontsector = line->sidedef[1]->sector;
+			sector_t *backsector = line->sidedef[0]->sector;
+			line->sidedef[0] = side1;
+			line->sidedef[1] = side2;
+			line->frontsector = frontsector;
+			line->backsector = backsector;
+		}
 	}
 	self->loader->ForceNodeBuild = true;
 	return 0;
diff --git a/wadsrc/static/zscript/level_postprocessor.zs b/wadsrc/static/zscript/level_postprocessor.zs
index 0ca307b7ef..9b3b7b7507 100644
--- a/wadsrc/static/zscript/level_postprocessor.zs
+++ b/wadsrc/static/zscript/level_postprocessor.zs
@@ -48,7 +48,7 @@ class LevelPostProcessor native play
 
 	protected native void SetVertex(uint vertex, double x, double y);
 	protected native void SetLineVertexes(uint Line, uint v1, uint v2);
-	protected native void SetLineSideRefs(uint Line, uint s1, uint s2);
+	protected native void FlipLineSideRefs(uint Line);
 	protected native void SetLineSectorRef(uint line, uint side, uint sector);
 	protected native Actor GetDefaultActor(Name actorclass);
 
@@ -59,13 +59,6 @@ class LevelPostProcessor native play
 		SetLineVertexes(Line, v2, v1);
 	}
 
-	protected void FlipLineSideRefs(uint Line)
-	{
-		uint s1 = level.lines[Line].sidedef[0].Index();
-		uint s2 = level.lines[Line].sidedef[1].Index();
-		SetLineSideRefs(Line, s2, s1);
-	}
-
 	protected void SetWallTexture(int line, int side, int texpart, String texture)
 	{
 		SetWallTextureID(line, side, texpart, TexMan.CheckForTexture(texture, TexMan.Type_Wall));