ASP.NET Create zip file for download: the compressed zipped

My problem seems only to be with the download. The code above executes without any issue. A file download dialog is presented. I can choose to either save or open. If I try to open the file from the dialog, or save it and then open it. I get the following dialog message:

The Compressed (zipped) Folder is invalid or corrupted.

my code

List<string> folderpath = new List<string>();
var zip = new ZipFile();
gridview.AllowPaging = false;
Response.ClearContent();
string zipName = String.Format("Zip_{0}.zip", DateTime.Now.ToString("yyyy-MMM-dd-HHmmss"));
Response.AddHeader("content-disposition", "attachment; filename=" + zipName);
Response.ContentType = "application/zip";
System.IO.StringWriter sw = new System.IO.StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);
gridview.RenderControl(htw);
//byte[] toBytes = Encoding.ASCII.GetBytes(zipName);
MemoryStream stream = new MemoryStream();
string attachment = sw.ToString();
byte[] data = Encoding.ASCII.GetBytes(attachment);
stream.Write(data, 0, data.Length);
stream.Seek(0, SeekOrigin.Begin);