diff --git a/src/maploader/postprocessor.cpp b/src/maploader/postprocessor.cpp
index 74e838f5fa..be339cb077 100644
--- a/src/maploader/postprocessor.cpp
+++ b/src/maploader/postprocessor.cpp
@@ -437,20 +437,23 @@ DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetVertex)
 	return 0;
 }
 
-DEFINE_ACTION_FUNCTION(DLevelPostProcessor, FlipLine)
+DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetLineVertexes)
 {
 	PARAM_SELF_PROLOGUE(DLevelPostProcessor);
 	PARAM_UINT(lineidx);
+	PARAM_UINT(vertexidx1);
+	PARAM_UINT(vertexidx2);
 
-	if (lineidx < self->Level->lines.Size())
+	if (lineidx < self->Level->lines.Size() &&
+		vertexidx1 < self->Level->vertexes.Size() &&
+		vertexidx2 < self->Level->vertexes.Size())
 	{
 		line_t *line = &self->Level->lines[lineidx];
-		vertex_t *v1 = line->v1;
-		vertex_t *v2 = line->v2;
-
-		line->v1 = v2;
-		line->v2 = v1;
+		vertex_t *vertex1 = &self->Level->vertexes[vertexidx1];
+		vertex_t *vertex2 = &self->Level->vertexes[vertexidx2];
 
+		line->v1 = vertex1;
+		line->v2 = vertex2;
 	}
 	self->loader->ForceNodeBuild = true;
 	return 0;
diff --git a/wadsrc/static/zscript/level_postprocessor.zs b/wadsrc/static/zscript/level_postprocessor.zs
index fb2ac6d957..3e3441d729 100644
--- a/wadsrc/static/zscript/level_postprocessor.zs
+++ b/wadsrc/static/zscript/level_postprocessor.zs
@@ -47,10 +47,17 @@ class LevelPostProcessor native play
 	protected native void SetThingStringArgument(uint thing, Name value);
 
 	protected native void SetVertex(uint vertex, double x, double y);
-	protected native void FlipLine(uint Line);
+	protected native void SetLineVertexes(uint Line, uint v1, uint v2);
 	protected native void SetLineSectorRef(uint line, uint side, uint sector);
 	protected native Actor GetDefaultActor(Name actorclass);
 
+	protected void FlipLine(uint Line)
+	{
+		uint v1 = level.lines[Line].v1.Index();
+		uint v2 = level.lines[Line].v2.Index();
+		SetLineVertexes(Line, v2, v1);
+	}
+
 	protected void SetWallTexture(int line, int side, int texpart, String texture)
 	{
 		SetWallTextureID(line, side, texpart, TexMan.CheckForTexture(texture, TexMan.Type_Wall));