Forms :: Calculations In Subform - Display Total Amount For All Records

I am trying to get the main form to display a total Amount for all records in Purchase order details but the control displays Error:

I have a Form called frmPurchaseOrder with a sub-form sbfrmPurchaseOrderDetails

sbfrmPurchaseOrderDetails takes item details from a table based on what is selected in the combo box then fills out four additional fields in the same sub-form.

SELECT tblItemListDetails.ItemID, tblItemListDetails.ItemName, tblItemListDetails.ItemSize, tblItemListDetails.ItemUnit, tblItemListDetails.ItemUnitCost, tblItemListDetails.ItemBrand FROM tblItemListDetails WHERE (((tblItemListDetails.Supplier)=[Forms]![frmPurchaseOrders]![SupplierCombo]));

Private Sub cmbItemName_Change()
Me.txtSize.Value = Me.cmbItemName.Column(2)

[Code] ....