上位?件の平均算出で Every derived table must have its own alias

mysqlの副問合せで、上位?件の平均を算出しようとしたら

select avg(val)
  from 
 (select val
  from measure_0_hour_day_201202
  where member_id=1
  order by val desc
  limit 3)
ERROR 1248 (42000): Every derived table must have its own alias

と、「きちんとエイリアスを示しなさい」と怒られた。
で、↓このように

 select avg(top3.val)
  from 
 (select val
  from measure_0_hour_day_201202
  where member_id=1
  order by val desc
  limit 3) top3
+---------------+
| avg(top3.val) |
+---------------+
|   4797.933333 |
+---------------+