I am trying to create a YAML template to extend from with a fixed stage “Deploy” (can not be modified by users, only pass params) and an optional stage “PreDeploy” that should be entirely provided by users (if they want to add this stage).

To achieve this I am using a stage parameter, but seems that my syntax is not correct since I get some error when saving:

Even when providing a fake stage as default parameter just to pass validation it still does not work:

What am I doing wrong?