async-graphql: Json scalar with a default value causes GraphQL Playground to fail the introspection query.

#[async_graphql::InputObject]
pub struct AssetInput {
    pub asset_type_id: ObjectId,
    pub data: Json<JsonMap>,
}

A definition like this results in

{
  "error": "Cannot convert value to AST: {}."
}

Queries work fine though, so I assume that this is some sort of problem with ObjectId Scalar definition.