WebRequest WebResponse sending receiveing XML Invalid Request

I don't what my problem is ..... This is in a code block of an ASPX page. I know the XML Im outputting is okay because I can save it to a file open in IE in the same folder that I've placed the dtd and is valid. However the web site Im talking to keeps returning an "Invalid XML Request" message. whats the deal?
 I've altered the code substantially for brevity...leaving the key components
 
  
WebRequest httpRequest = System.Net.WebRequest.Create("https://url");
httpRequest.Method = "POST";
httpRequest.ContentType = "text/xml";

try
{

System.Xml.XmlTextWriter xmlWriter = new System.Xml.XmlTextWriter(httpRequest.GetRequestStream(), System.Text.Encoding.UTF8);

xmlWriter.WriteStartDocument();
...