[AccessD] populating a textbox-type field in one table, calc'ed from another table

Dear Colleagues,

The below expression works when the display control for the field is a
listbox, and the above expression its row source. But it won't work (as
written) when the display control is a textbox and the expression is the
Default Value -- getting syntax error.

Default Value = SELECT Sum(Tbl_Field_Values.Items) FROM Tbl_Field_Values
WHERE Tbl_Field_Values.WB = WB  and Tbl_Field_Values.WS = WS  and
Tbl_Field_Values.FLD = FLD GROUP BY Tbl_Field_Values.WB,
Tbl_Field_Values.WS, Tbl_Field_Values.FLD;

Table: Tbl_Field_Values_Header
Field: SumOfItems


Can the syntax be improved to cause the sum of all items where WB, WS, and
FLD match between Tbl_Field_Values and Tbl_Field_Values_Header, to display
as the default value in the field I am trying to populate (SumOfItems) in my
Tbl_Field_Values_Header table?