Setting URL on a WebBrowser object

Question

Here is my code in our SetPreview(string previewURL) method webBrowser1.Url = new Uri(previewUrl); where the previewURLstring is something like this: "http://companyname.com/shows/AppVideos/Videos/video.mp4" When I get past this statement the webBrowser1.Url = null. Can anyone tell me why? Thanks