aggregate calculation works in select query but not an update quer

The below query works well as a Select Query. But when I turn it into an
update query, I get a whole lot of nothing in the results.

Interestingly, if I try to call this query from a Macro on my form, I get
error (-20324) repeated the number of times that there are records in the
query.

It was designed to go into a table of price breaks & decide whether the
quantities being ordered meet the price breaks. It then takes either that
adjusted price, or the original price & populate the values into a table.

Select Query:
SELECT QryBOMDetails2.ParentBOMID, QryBOMDetails2.BOMPrice,
IIf(IsNull(DMin("[Price]","TblMfrPriceBreak","[MfrPNID]=" & [MfrPNID] & "And
[Level]=" &
[ExtQty])),[mfrbaseprice],DMin("[Price]","TblMfrPriceBreak","[MfrPNID]=" &
[MfrPNID] & "And [Level]=" & [ExtQty])) AS q
FROM QryBOMDetails2
WHERE (((QryBOMDetails2.ParentBOMID)=[Forms]![FrmBOMEntry]![TxtBOMID]) AND
((QryBOMDetails2.BOMPrice) Is Null));

Update Query:
UPDATE QryBOMDetails2 SET QryBOMDetails2.BOMPrice =
IIf(IsNull(DMin("[Price]","TblMfrPriceBreak","[MfrPNID]=" & [MfrPNID] & "And
[Level]=" &
[ExtQty])),[mfrbaseprice],DMin("[Price]","TblMfrPriceBreak","[MfrPNID]=" &
[MfrPNID] & "And [Level]=" & [ExtQty]))
WHERE (((QryBOMDetails2.BOMPrice) Is Null) AND
((QryBOMDetails2.ParentBOMID)=[Forms]![FrmBOMEntry]![TxtBOMID]));

FYI I'm doing this using the query builder.
Thanks in advance for any advice.