diff --git a/chapter-20/resources/shaders/scene.vert b/chapter-20/resources/shaders/scene.vert
index 0aab496..ca0e65e 100644
--- a/chapter-20/resources/shaders/scene.vert
+++ b/chapter-20/resources/shaders/scene.vert
@@ -15,7 +15,7 @@ out vec3 outBitangent;
 out vec2 outTextCoord;
 out vec4 outViewPosition;
 out vec4 outWorldPosition;
-out uint outMaterialIdx;
+flat out uint outMaterialIdx;
 
 struct DrawElement
 {
diff --git a/chapter-21/resources/shaders/scene.vert b/chapter-21/resources/shaders/scene.vert
index 0aab496..ca0e65e 100644
--- a/chapter-21/resources/shaders/scene.vert
+++ b/chapter-21/resources/shaders/scene.vert
@@ -15,7 +15,7 @@ out vec3 outBitangent;
 out vec2 outTextCoord;
 out vec4 outViewPosition;
 out vec4 outWorldPosition;
-out uint outMaterialIdx;
+flat out uint outMaterialIdx;
 
 struct DrawElement
 {