diff --git a/test/plugin/test_out_forward.rb b/test/plugin/test_out_forward.rb
index 173dfc9efb..e3248a782a 100644
--- a/test/plugin/test_out_forward.rb
+++ b/test/plugin/test_out_forward.rb
@@ -530,6 +530,8 @@ def test_heartbeat_type_none
     assert_equal node.available, true
   end
 
+  # To suppress calling `ForwardInput#start` in `DummyEngineDriver`,
+  # `DummyEnigneDriver` should avoid calling CallSuperMixin.prepend at `Fluent::Compat::Input#initialize`.
   module SuppressCallSuperMixin
     def start
       _start