Help with: ...query that does not include specified expression ....as part of an aggregate function...

Good Afternoon,

I keep getting "You tried to execute a query that does not include the
specified expression '(Max([b].[AS OF DATE])) Between [a].[closedate]
And [a].[trade date]' as part of an aggregate function." when runnng
the query below. Any assistance offered is greatly appreciated.
Thank you.

UPDATE DISTINCTROW tbl_TXN_PortIncExp AS a LEFT JOIN tbl_HLD AS b ON
(a.[CUSIP OR LOAN #] = b.[CUSIP OR LOAN #]) AND (a.[PORTFOLIO CODE] =
b.[PORTFOLIO CODE])
SET a.XDate = (Max([b].[AS OF DATE])) Between [a].[closedate] And [a].
[trade date], a.TRANSTYPE = "Income"
WHERE (((a.TRANSTYPE)="portincome"));