Vista - File Downloads

I have a simple file download button that when clicked opens a window with a
file download dialog prompting a user to save in csv or xls format. The code
works fine locally. (In debug mode there is an exception thrown on the
response.end line) but once on the server the page displays and then closes
immediately. Below is the relevant code. Has anyone encountered this before?

Response.Clear()
HttpContext.Current.Response.BufferOutput = True
HttpContext.Current.Response.AppendHeader("Content-Disposition",
String.Format("attachment; filename={0}", fileName))
HttpContext.Current.Response.ContentType = outType
Dim encoding As New System.Text.ASCIIEncoding
HttpContext.Current.Response.BinaryWrite(encoding.GetBytes(outString))
HttpContext.Current.Response.Flush()
HttpContext.Current.Response.End()