Help : Nested Sql Statement

I am trying to run this simple nested sql statement, just wondering what am i doing wrong.

Appreciate any help


SELECT dtl_doccode, dtl_prodid, sum(TotalItems)
FROM
(
SELECT
dtl_doccode,
dtl_prodid,
dtl_prodquantity,
uomd_packing,
TotalItems = dtl_prodquantity * uomd_packing
FROM tblDocumentDetails
Inner Join tblProducts ON tblDocumentDetails.DTL_ProdId = tblProducts.PROD_Id
Inner Join tblDocuments ON tblDocumentDetails.DTL_DocId = tblDocuments.DOC_Id
Inner Join tbluomdetails On tblDocumentDetails.dtl_produomdid = uomd_id
WHERE
tblDocumentDetails.DeletedFlag = 0
AND DOC_TypeId = 20
AND DTL_DocId = 451
AND PROD_Id = '17'
)
GROUP BY dtl_doccode, dtl_prodid