Passing Session Info To Popup

I have a report number that is generated from a page, and I want to display that number in a popup window upon a button click that saves the report to a database. On the original page, here's the code that I am trying to use:

Code:
Private Sub saverpt()
Dim url As String = "popup.aspx"
Dim s As String = "window.open('" & url + "', 'popup_window', 'width=300,height=100,left=100,top=100,resizable=yes');"
ClientScript.RegisterStartupScript(Me.GetType(), "script", s, True)
Session("rptnum") = rptnum.Text

On the popup.aspx page, here's what I am trying to make the rptnum field appear on:

Code:
Private Sub popup_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim rptnum As String = CType(Session.Item("rptnum"), String)
Label1.Text = rptnum.text
End Sub

Currently, the popup will display "rptnum" rather than the actual generated report number.