diff --git a/flow/layers/opacity_layer.h b/flow/layers/opacity_layer.h
index e437041c21f..93f73988d9e 100644
--- a/flow/layers/opacity_layer.h
+++ b/flow/layers/opacity_layer.h
@@ -9,6 +9,10 @@
 
 namespace flutter {
 
+// Don't add an OpacityLayer with no children to the layer tree. Painting an
+// OpacityLayer is very costly due to the saveLayer call. If there's no child,
+// having the OpacityLayer or not has the same effect. In debug_unopt build, the
+// |EnsureSingleChild| will assert if there are no children.
 class OpacityLayer : public ContainerLayer {
  public:
   OpacityLayer();