diff --git a/src/gl/scene/gl_portal.cpp b/src/gl/scene/gl_portal.cpp
index a22c25e2d1..b5ef31e366 100644
--- a/src/gl/scene/gl_portal.cpp
+++ b/src/gl/scene/gl_portal.cpp
@@ -291,7 +291,6 @@ bool GLPortal::Start(bool usestencil, bool doquery)
 	planestack.Push(gl_RenderState.GetClipHeightTop());
 	planestack.Push(gl_RenderState.GetClipHeightBottom());
 	glDisable(GL_CLIP_DISTANCE0);
-	glDisable(GL_CLIP_DISTANCE1);
 	gl_RenderState.SetClipHeightBottom(-65536.f);
 	gl_RenderState.SetClipHeightTop(65536.f);
 
@@ -360,7 +359,7 @@ void GLPortal::End(bool usestencil)
 	if (f > -65535.f) glEnable(GL_CLIP_DISTANCE0);
 	planestack.Pop(f);
 	gl_RenderState.SetClipHeightTop(f);
-	if (f < 65535.f) glEnable(GL_CLIP_DISTANCE1);
+	if (f < 65535.f) glEnable(GL_CLIP_DISTANCE0);
 
 	if (usestencil)
 	{
@@ -808,17 +807,17 @@ void GLPlaneMirrorPortal::DrawContents()
 	if (PlaneMirrorMode < 0)
 	{
 		gl_RenderState.SetClipHeightTop(f);	// ceiling mirror: clip everything with a z lower than the portal's ceiling
-		glEnable(GL_CLIP_DISTANCE1);
+		gl_RenderState.SetClipHeightBottom(-65536.f);
 	}
 	else
 	{
 		gl_RenderState.SetClipHeightBottom(f);	// floor mirror: clip everything with a z higher than the portal's floor
-		glEnable(GL_CLIP_DISTANCE0);
+		gl_RenderState.SetClipHeightTop(65536.f);
 	}
 
+	glEnable(GL_CLIP_DISTANCE0);
 	GLRenderer->DrawScene();
 	glDisable(GL_CLIP_DISTANCE0);
-	glDisable(GL_CLIP_DISTANCE1);
 	gl_RenderState.SetClipHeightBottom(-65536.f);
 	gl_RenderState.SetClipHeightTop(65536.f);
 	PlaneMirrorFlag--;
diff --git a/wadsrc/static/shaders/glsl/main.vp b/wadsrc/static/shaders/glsl/main.vp
index 75f814887b..0aef2d35af 100644
--- a/wadsrc/static/shaders/glsl/main.vp
+++ b/wadsrc/static/shaders/glsl/main.vp
@@ -45,8 +45,14 @@ void main()
 	gl_Position = ProjectionMatrix * eyeCoordPos;
 	
 	// clip planes used for reflective flats
-	gl_ClipDistance[0] = worldcoord.y - uClipHeightBottom;
-	gl_ClipDistance[1] = uClipHeightTop - worldcoord.y;
+	if (uClipHeightBottom  > -65536.0)
+	{
+		gl_ClipDistance[0] = worldcoord.y - uClipHeightBottom;
+	}
+	else if (uClipHeightTop < 65536.0)
+	{
+		gl_ClipDistance[0] = uClipHeightTop - worldcoord.y;
+	}
 	
 	gl_ClipDistance[2] = worldcoord.y - uClipSplit.x;
 	gl_ClipDistance[3] = uClipSplit.y - worldcoord.y;