diff --git a/src/gl/compatibility/gl_20.cpp b/src/gl/compatibility/gl_20.cpp
index da1d698ba..367a627d2 100644
--- a/src/gl/compatibility/gl_20.cpp
+++ b/src/gl/compatibility/gl_20.cpp
@@ -747,7 +747,7 @@ bool GLWall::PrepareLight(ADynamicLight * light, int pass)
 	float scale;
 
 	auto normal = glseg.Normal();
-	p.Set(normal, -normal.X * glseg.x1 - normal.Y * glseg.y1);
+	p.Set(normal, -normal.X * glseg.x1 - normal.Z * glseg.y1);
 
 	if (!p.ValidNormal())
 	{
diff --git a/src/gl/scene/gl_walls_draw.cpp b/src/gl/scene/gl_walls_draw.cpp
index 0fb2e38bf..a71053217 100644
--- a/src/gl/scene/gl_walls_draw.cpp
+++ b/src/gl/scene/gl_walls_draw.cpp
@@ -73,7 +73,7 @@ void GLWall::SetupLights()
 	lightdata.Clear();
 
 	auto normal = glseg.Normal();
-	p.Set(normal, -normal.X * glseg.x1 - normal.Y * glseg.y1);
+	p.Set(normal, -normal.X * glseg.x1 - normal.Z * glseg.y1);
 
 	FLightNode *node;
 	if (seg->sidedef == NULL)