Slot_was_not_set was not defined error

I got this error when I tried to used this marker as described in the docs Markers

YamlValidationException: Failed to validate ‘data\stories.yml’. Please make sure the file is correct and all mandatory parameters are specified. Here are the errors found during validation: in data\stories.yml:40: Key ‘slot_was_not_set’ was not defined. Path: ‘/stories/6/steps/1’

Is there something wrong with my rasa install or something?

@ooferdoodles: Any update or solution ? I’m struggling with the same problem right now…