TransmitFile downloads my .aspx page to the client!!

Hil,
I have written the following code to download a dynamically created .doc file into client. The file is downloaded perfectly when the website is running under Visual Studio's web server. But under IIS, my .aspx page is downloaded !! I mean when I open the downloaded .DOC file, it includes my .aspx page !
                Response.Buffer = false;
                Response.Clear();
                Response.AddHeader("content-disposition", "attachment;filename=" + FileName);
                Response.ContentType = "Application/msword";
                ...