Ah yeah the space is not obvious given that the help message wraps the line just after the :. That tricked me recently as well. The error message has been improved recently to make it clearer that a YAML dictionary is expected.
This has been brought up recently but it hasn’t been decided yet if metapackages should exist as ament packages or if they should be stored in a central place given that they are for producing binaries only and should not be used as dependencies