Dim MyRec As Variant, MyListB As Control
    Dim MySum As Double
    Dim Source As String
    Dim rst As ADODB.Recordset
    Set MyListB = Forms!MyForm!List1
    Source =  ""
    For Each MyRec In MyListB.ItemsSelected
        Source = Source & " ,  " & MyListB.ItemData(ManoSask)
    Next
    Source = " SELECT SUM([Summa]) AS Summa FROM Table1 WHERE [ID] IN ( " & _
             Left(Source, Len(Source) - 1 ) & ") "
    Set rst = CurrentProject.Connection.Execute(Source)
    MySum = rst.Fields(" Summa").Value
    rst.Close
    MsgBox MySum 'Теперь будет работать нормально