Queries :: DSum Always One Record Behind

I am using DSum to total some records located in a subform. I have code to save the value returned by DSum to a table on a lost focus event. It works great except for one thing. The value saved is always one record behind. The save code is:

Dim dbs As dao.Database
Dim rst As dao.Recordset
Set dbs = CurrentDb
Set rst = dbs.OpenRecordset("tblInvoice", dbOpenDynaset)
rst.FindFirst "ID = " & Forms!frmInvoice!txtID
With rst
.Edit
!Total = Forms!frmInvoice!txtTotal
.Update
End With
Exit Sub

txtTotal is the textbox containing the DSum value. It displays the proper value. If I insert a blank row the routine will finally pass the right value. I tried running the code from a button but it still copies the old total.