diff --git a/upgrading-to-drupal-9/slides.rst b/upgrading-to-drupal-9/slides.rst
index 9faf64e..d4e72f6 100644
--- a/upgrading-to-drupal-9/slides.rst
+++ b/upgrading-to-drupal-9/slides.rst
@@ -401,6 +401,7 @@ Fixing autowiring
       services:
     +   Drupal\Core\Messenger\MessengerInterface:
     +     alias: messenger
+    +     private: true
 
         Drupal\simple_message\DisplaySimpleMessage:
           autowire: true