The following sub routine produces two errors:

Private Sub Form_Open(Cancel As Integer)

    PODetailUpdate
    
End Sub

Private Sub POList_AfterUpdate()

    PODetailUpdate
    
End Sub

Private Sub PODetailUpdate()

    If IsNull(POList) Then
        'no PO selected - show no records
        PurchaseOrderDetailsF.Form.RecordSource = "Select * From PurchaseOrderDetailsF Where PurchaseOrderID=-1"
    Else
        'a PO selected, show just those records
        PurchaseOrderDetailsF.Form.RecordSource = "Select * From PurchaseOrderDetailsF Where PurchaseOrderID=" & POList
    End If
    

End Sub

When the sub runs in the after update event, the following error is reported:
The recordsource 'Select * From PurchaseOrderDetailsF Where PurchaseOrderID=1' specified on this form or report does not exist

When the sub routine runs from the OnOpen Event, the following error is reported:
The recordsource 'Select * From PurchaseOrderDetailsF Where PurchaseOrderID=-1' specified on this form or report does not exist.

What is my problem?