diff --git a/src/engine/renderer/tr_bsp.cpp b/src/engine/renderer/tr_bsp.cpp
index 09b43f06f..3f6d39e53 100644
--- a/src/engine/renderer/tr_bsp.cpp
+++ b/src/engine/renderer/tr_bsp.cpp
@@ -6728,6 +6728,7 @@ void R_BuildCubeMaps()
 
 			int msecUnused1;
 			int msecUnused2;
+			// Material system writes culled surfaces for the next frame, so we need to render twice with it to cull correctly
 			if ( glConfig2.materialSystemAvailable ) {
 				tr.refdef.pixelTarget = nullptr;
 
diff --git a/src/engine/renderer/tr_scene.cpp b/src/engine/renderer/tr_scene.cpp
index 65351e926..1a8b1a1b5 100644
--- a/src/engine/renderer/tr_scene.cpp
+++ b/src/engine/renderer/tr_scene.cpp
@@ -769,6 +769,7 @@ void RE_RenderScene( const refdef_t *fd )
 
 	if ( showCubeProbe ) {
 		if ( glConfig2.materialSystemAvailable ) {
+			// Material system writes culled surfaces for the next frame, so we need to render twice with it to cull correctly
 			R_SyncRenderThread();
 			RE_RenderScene( &refdef );
 		}