diff --git a/flow/layers/opacity_layer.cc b/flow/layers/opacity_layer.cc
index 436a885..99c705d 100644
--- a/flow/layers/opacity_layer.cc
+++ b/flow/layers/opacity_layer.cc
@@ -20,6 +20,13 @@
   }
 
   auto new_child = std::make_shared<flow::TransformLayer>();
+
+  // Be careful: SkMatrix's default constructor doesn't initialize the matrix to
+  // identity. Hence we have to explicitly call SkMatrix::setIdentity.
+  SkMatrix identity;
+  identity.setIdentity();
+
+  new_child->set_transform(identity);
   for (auto& child : layers()) {
     new_child->Add(child);
   }
diff --git a/flow/layers/transform_layer.h b/flow/layers/transform_layer.h
index b1fb830..ea52506 100644
--- a/flow/layers/transform_layer.h
+++ b/flow/layers/transform_layer.h
@@ -9,6 +9,8 @@
 
 namespace flow {
 
+// Be careful that SkMatrix's default constructor doesn't initialize the matrix
+// at all. Hence |set_transform| must be called with an initialized SkMatrix.
 class TransformLayer : public ContainerLayer {
  public:
   TransformLayer();