You have specified a default value for a type that cannot be explicitly converted to an Ecto default

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Honestly, the EctoMigrationDefault isn’t the right answer here. You’d have to implement the protocol for arrays generically. We’re kind of moving away from EctoMigrationDefault in general anyway. I would do this:

postgres do
   migration_defaults [allowed_types: "[:value_add, :non_value_add]"]
end

EDIT: sorry, just saw your footnote. I’d advise against using EctoMigrationDefault in general unless the value you’re using it for is a struct (like Money) and the type is not in your control (because if it is there is a callback you can use to set the default value).