Artemis code generation failing to build a Dart class for one of the types in my GraphQL schema

This is an excerpt from a schema I’m working on. I’m trying to use Artemis to generate Dart classes for a Flutter app.

interface Node {
  """The ID of the object"""
  id: ID!
}

type PictureNode implements Node {
  """The ID of the object"""
  id: ID!
  image: String!
  created: DateTime!
  thumbs: ThumbnailsNode
}

type ThumbnailsNode {
  sd: String
  smallSquare: String
}

Running dart run build_runner build results in this following error:

[SEVERE] json_serializable on lib/graphql/schema.graphql.dart:

Could not generate `fromJson` code for `thumbs`.
To support the type `InvalidType` you can:
* Use `JsonConverter`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
* Use `JsonKey` fields `fromJson` and `toJson`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html
package:treeminder_client/graphql/schema.graphql.dart:70:7
   ╷
70 │       thumbs;
   │       ^^^^^^
   ╵

What is confusing to me, is that the generator is able to generate the classes including serialziation/deserialization methods for other types in the schema, but it is tripping up on this one, and I don’t understand the difference. When I remove the thumbs field from PictureNode, and the relevant queries, the codegenerator runs fine. I assume I’m overlooking something very simple here.