httpwebrequest和httpwebresponse 报错!请求帮助。急！！

Private Function ResponseRequest(ByVal URL As String, ByVal Par As String) As String
        Dim Resp As HttpWebResponse
        Dim Req As HttpWebRequest
        Dim stream() As Byte
        Dim postream As Stream
        Dim reader As StreamReader
        Dim xml As String
        'Dim URI As New Uri(portal & URL)
        Dim URI As New Uri(URL)
        Req = CType(HttpWebRequest.Create(URI), HttpWebRequest)
        Req.Method = "post"
        Req.Timeout = 18000
        Req.ContentType = "application/x-www-form-urlencoded"
        stream = Encoding.ASCII.GetBytes(Par)
        Req.ContentLength = stream.Length        postream = Req.GetRequestStream()'执行到此处时有错
        postream.Write(stream, 0, stream.Length)
        postream.Close()
        Resp = CType(Req.GetResponse, HttpWebResponse)
        reader = New StreamReader(Resp.GetResponseStream, Encoding.GetEncoding("GBK"))
        xml = reader.ReadToEnd
        Resp.Close()
        reader.Close()
        reader = Nothing
        URI = Nothing
        xml = xml.Replace(vbCrLf, "").Trim
        Return xml
    End Function报错提示==无法发送具有此谓词类型的内容正文。