PLS-00103: 出现符号 SELECT在需要下列之一时 <an identifier><a double-quoted delimited-identifier>

编译错误：
PLS-00103: 出现符号 SELECT在需要下列之一时 <an identifier><a double-quoted delimited-identifier>

错误语句：

UPDATE b
SET minitqty =
( minitqty
+ (SELECT trxqtysum ---报错行
FROM a
WHERE b.wno = a.wno
AND b.mcd = a.mcd
AND TRIM (b.wno) = TRIM (vs_wno))
)
WHERE EXISTS (
SELECT 'X'
FROM a
WHERE b.wno = a.wno
AND b.mcd = a.mcd
AND TRIM (b.wno) = TRIM (vs_wno));

说明：其中wno，mcd为a表的主键，确定a表唯一记录行。