List of Object makes an error (Tried to treat object as if it were leaf field)

[nishirken]

this code:

type Friend = Object "Friend" '[]
  '[ Field "id" Text.Text, Field "email" Text.Text ]

type User = Object "User" '[]
  '[ Field "id" Text.Text, Field "email" Text.Text, Field "friends" (List Friend) ]

type UserQuery = Object "UserQuery" '[]
  '[ Argument "id" Text.Text :> Field "user" (Maybe User) ]

friendHandler :: DbUser.DbUser -> Handler IO Friend
friendHandler DbUser.DbUser {..} = pure $ pure ((Text.pack . show) _id) :<> pure _email

friendsHandler :: [DbUser.DbUser] -> Handler IO (List Friend)
friendsHandler friends = pure $ map friendHandler friends

userHandler :: Int -> Text.Text -> [DbUser.DbUser] -> Handler IO User
userHandler id email friends = pure $
  pure ((Text.pack . show) id) :<> pure email :<> friendsHandler friends

userQueryHandler :: PSQL.Connection -> Handler IO UserQuery
userQueryHandler dbConn = pure $ \userId -> do
  result <- Db.getUserById dbConn (read @Int $ Text.unpack userId)
  case result of
    [DbUser.DbUser {..}] -> do
      friends <- Db.getUsersByIds dbConn _friendsIds
      pure $ Just $ userHandler _id _email friends
    _ -> pure Nothing

and query:

{
  user(id: "1") {
    id
    email
    friends
  }
}

compiled successfully, but with errors in a response:

{
  "data": {
    "user": {
      "email": "test@mail.com",
      "id": "1",
      "friends": [
        null
      ]
    }
  },
  "errors": [
    {
      "message": "Tried to treat object as if it were leaf field."
    }
  ]
}

But if i change type

type User = Object "User" '[]
  '[ Field "id" Text.Text, Field "email" Text.Text, Field "friends" (List Text.Text) ]

everything works

So, what I did wrong and what this error means? Thanks for any help.

ghc - 8.6.5
graphql-api-0.3.0