diff --git a/lib/racecar/instrumenter.rb b/lib/racecar/instrumenter.rb
index <HASH>..<HASH> 100644
--- a/lib/racecar/instrumenter.rb
+++ b/lib/racecar/instrumenter.rb
@@ -11,6 +11,7 @@ module Racecar
       @default_payload = default_payload
 
       @backend = if defined?(ActiveSupport::Notifications)
+        # ActiveSupport needs `concurrent-ruby` but doesn't `require` it.
         require 'concurrent/utility/monotonic_time'
         ActiveSupport::Notifications
       else
issue #<I>: comment why monotonic_time is required