Over（order by）窗口函数不起作用

Question

我正在运行以下查询

SELECT fdPaymentDate,
   COUNT(fnAccNo ) as count,
   SUM(COUNT(fnAccNo )) OVER (ORDER BY fdPaymentDate) as running_total
FROM tblPaymentDetail
GROUP BY fdPaymentDate
ORDER BY fdPaymentDate;

它给出了错误 'order'附近的语法不正确。

请帮我解决这个问题