The archive is either in unknown format or damaged when zipping a file in asp.net

protected void btn_downloaddocuments_Click(object sender, EventArgs e)
{

//ExportFunction();
var filePath = string.Empty;
var fileLocation = string.Empty;

//string downloadFileName = "Files.zip";

Response.ContentType = "application/octet-stream";

Response.ContentType = "application/download";




Response.AddHeader("Content-Disposition", "attachment; filename=DownloadedFile.zip");

//Content-Transfer-Encoding: base64
Response.Cache.SetCacheability(HttpCacheability.NoCache);

Response.Charset = "";
// Response.AddHeader("content-disposition", "filename=" + downloadFileName);
var ms = new MemoryStream();
var fileLocation1 = "";
using (ZipFile zip = new ZipFile())
{

foreach (GridViewRow row in gvdownloaddocuments.Rows)
{


CheckBox cb = (CheckBox)row.FindControl("CheckBox1");
//if (cb != null && cb.Checked)
//{
filePath = (row.FindControl("lblFileName") as Label).Text;
fileLocation = Server.MapPath("~/Document Images/" + filePath);




// zip.AddFile(fileLocation);
zip.AddFile(fileLocation, @"\Document Images");
// zip.AddFile(fileLocation);

fileLocation1 = Server.MapPath("~/Document Images/" + "DownloadedFiles");


}

zip.Save(fileLocation1 + ".zip");


// zip.Save(fileLocation1 + ".zip");
// zip.Save(Response.OutputStream);
ms.Position = 0;
ms.CopyTo(Response.OutputStream);
//Response.End();
// Response.End();

// Response.Close();



}
HttpContext.Current.Response.End();
// Response.End();

//Response.Close();


}

...This is the code which i have used so far..But when i zip any file..it is throwing a error like...(The-archive-is-either-in-unknown-format-or-damaged)