From subquery giving error in oracle

I am having this query :

SELECT T.custno,T.custlastname,AVG(T.OrderAmount) , T.OrderCount
FROM(
      SELECT  A.custno,A.custlastname,count(b.ordno) as OrderCount,    sum(c.qty*d.prodprice) AS  OrderAmount
  FROM customer A 
      JOIN ordertbl B ON A.custno=b.custno
      JOIN ordline C ON b.ordno=c.ordno
      JOIN  product D ON c.prodno=d.prodno
  WHERE A.custstate='CO'
  GROUP BY A.custno,A.custlastname, b.ordno) AS T
GROUP BY T.custno,T.custlastname;

I get this error :

ORA-00933: SQL command not properly ended

When i execute inner subquery explicitly, it runs fine. Please let me know the reason.

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via Chebli Mohamed