c# - code for downloading documents in zip is not working -

i have written method download files in zip folder. after downloading when opening file it's giving error-"an attempt made move pointer before beginning of file." using ionic zip. code is-

 protected void zipdownload()     {         var list = db.documents.where(u => u.userid == (int)session["usrid"]).select(u => new { u.doc, u.docname, u.doctype });         zipfile zip = new zipfile();         foreach (var file in list)         {              zip.addentry(file.docname, (byte[])file.doc.toarray());         }         var zipms = new memorystream();         zip.save(zipms);         byte[] filedata = zipms.getbuffer();         zipms.seek(0, seekorigin.begin);         zipms.flush();         response.clear();         response.addheader("content-disposition", "attachment;filename=docs.zip ");         response.contenttype = "application/zip";         response.binarywrite(filedata);         response.end();      }