diff --git a/wadsrc/static/shaders/lightmap/vert_copy.glsl b/wadsrc/static/shaders/lightmap/vert_copy.glsl
index 9e2643667..c73b6b6de 100644
--- a/wadsrc/static/shaders/lightmap/vert_copy.glsl
+++ b/wadsrc/static/shaders/lightmap/vert_copy.glsl
@@ -15,7 +15,7 @@ struct TileCopy
 	int Padding1, Padding2;
 };
 
-layout(std430, set = 0, binding = 1) buffer CopyBuffer
+layout(std430, set = 0, binding = 1) buffer readonly CopyBuffer
 {
 	TileCopy tiles[];
 };