我定义了一个PostgreSQL表，如下所示：

> \d+ public."Users"
                                                                   Table "public.Users"
   Column    |           Type           | Collation | Nullable |               Default               | Storage  | Compression | Stats target | Description 
-------------+--------------------------+-----------+----------+-------------------------------------+----------+-------------+--------------+-------------
 id          | integer                  |           | not null | nextval('"Users_id_seq"'::regclass) | plain    |             |              | 
 username    | character varying(255)   |           | not null |                                     | extended |             |              | 
 userId      | character varying(255)   |           |          |                                     | extended |             |              |

我尝试使用postgresql-simple查询该表：

maybeUsername <- liftIO (listToMaybe <$> query conn "SELECT username FROM public.\"Users\" WHERE \"userId\" = ?" (Only userId) :: IO (Maybe String))
  case maybeUsername of
    Just username -> liftIO $ putStrLn $ "Username is " ++ username
    Nothing -> liftIO $ putStrLn $ "Could not find a record for userId " ++ userId

由于某种原因，我得到了以下错误：

Incompatible {errSQLType = "varchar", errSQLTableOid = Just (Oid 24782), errSQLField = "username", errHaskellType = "Char", errMessage = ""}

有办法解决吗？