You're trying to assign an object property to a string. Declare the variable as an object, not a string - in this case, a control. Try
Code:
Private Sub cboInvoiceMonth_Exit(Cancel As Integer)
Dim ctl As Control
Set ctl = [Forms]![frmNavMain]![frmNavMain].[Form]![frmNavInvoices].[Form].lstSub
ctl.RowSource = "qrySub_Lost"
End Sub
EDIT - if you need to, set a variable to a form so that your reference is to the parent object. That way you can use the same variable when referencing several controls on the form.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.