@@ -49,6 +49,9 @@ argument of type ``service_closure``:
 App\Service\MyService:
 arguments: [!service_closure '@mailer']
 
+# In case the dependency is optional
+# arguments: [!service_closure '@?mailer']
+
  .. code-block:: xml
 
 <!-- config/services.xml -->
@@ -60,6 +63,11 @@ argument of type ``service_closure``:
 <services>
 <service id="App\Service\MyService">
 <argument type="service_closure" id="mailer"/>
+
+<!--
+In case the dependency is optional
+<argument type="service_closure" id="mailer" on-invalid="ignore"/>
+-->
 </service>
 </services>
 </container>