Help Crystal Report filter empty records

[dicky18]

hey there im new to crystal reports and i need help in filtering empty records for example i have an invoice with a PayDate as a record
i want to filter the records that are empty no date in there
i mean i want to see only the not paid records when the date is empty
i have this code
Dim Fltr
Dim str As String
Dim order As String

order = "{CustomerOrder.AmoundDue} <----- i dont know what to put here

str = str + order
Me.CrystalReportViewer1.ReportSource = ".\CustomerInvoice.rpt"
Me.CrystalReportViewer1.SelectionFormula = str
Me.CrystalReportViewer1.RefreshReport()

or any code could work
thanks