diff --git a/src/mol-gl/shader/direct-volume.frag.ts b/src/mol-gl/shader/direct-volume.frag.ts
index 45c16ede79c260b77b75589b241a7733c3581144..88cdd09e8338deacdce084ec78d4e48cde7ce51e 100644
--- a/src/mol-gl/shader/direct-volume.frag.ts
+++ b/src/mol-gl/shader/direct-volume.frag.ts
@@ -74,6 +74,7 @@ uniform vec3 uFogColor;
 uniform float uAlpha;
 uniform float uPickingAlphaThreshold;
 uniform bool uTransparentBackground;
+uniform float uXrayEdgeFalloff;
 
 uniform float uInteriorDarkening;
 uniform bool uInteriorColorFlag;