Crystal Reports in ASPX page

Can anybody help on how to place a crystal report viewer control and reportdocument control on an aspx page and pass the report name dynamically.

I've tried placing the Crystal Viewer control and report document control on the page and in page load I've used the following code

Code:

---


        reportDocument1.Load(MapPath("CrystalReport1.rpt"))
        reportDocument1.SetDataSource(DsEx)
        CrystalReportViewer1.ReportSource = reportDocument1

---

after filling the dataset(DsEx) with the Dataapdapter.

However I'm getting a nullreference exception.

Pls. Help


Thnx in Advance