Well,

It's not an error first of all.
The viewer control has two events of interest to you.
The first one : DownloadStarted, triggers when the viewer starts retreiving the data for the report you want to view.
The other one, DownloadFinished, when it's finished loading the data.
In the DownloadFinished-event, you should trigger the ViewReport() method, not before.

Is this of any help to you?

Greetz

Guillaume