Links become undefined after file download in Firefox/Chrome/Safari, not IE

Question

Using ASP.NET 2

I have an aspx page where a linkbutton posts back, generates a pdf file, and uses the following method to send the file back to the client

  Dim r As System.Web.HttpResponse = System.Web.HttpContext.Current.Response
  r.Clear()
  r.ClearContent()
  r.ClearHeaders()

  r.AppendHeader("content-disposition", "attachment; filename=" & userFileName)
  r.ContentType = "application/pdf"

  r.TransmitFile(filePath)

  r.Flush()
  r.End()

This causes an Open/Save dialog box to pop open, which is exactly what I want.

However, after saving the file, ALL the links in the page still in the browser are broken. For example, a link that WAS

http://www.myserver.com/myapp/admin/commissionmgmt.aspx

becomes

http://www.myserver.com/myapp/admin/undefined

It is even changing the directory structure of the link. A link that was:

http://www.myserver.com/myapp/home.aspx

becomes

http://www.myserver.com/myapp/admin/undefined

This is the case for EVERY link on the page

This happens (on windows xp) in Safari, Chrome and Firefox (all most recent versions) but NOT in IE. Most of my users use IE but I'd like this to work everywhere.