webRequest的Post方法发送xml串时，提示错误：（500）

Public Shared Function getPage(ByVal url As String, ByVal sData As String) As String
        Dim result As WebResponse        Try
            Dim encode As Encoding = System.Text.Encoding.GetEncoding("gb2312")            Dim req As WebRequest
            Dim RequestStream As Stream
            Dim ReceiveStream As Stream
            Dim sr As StreamReader            req = WebRequest.Create(url)
            req.Method = "POST"
            req.ContentType = "application/x-www-form-urlencoded"
            Dim SomeBytes() As Byte            If sData <> "" Then
                SomeBytes = encode.GetBytes(sData)                req.ContentLength = SomeBytes.Length
                RequestStream = req.GetRequestStream()
                RequestStream.Write(SomeBytes, 0, SomeBytes.Length)
                RequestStream.Close()            Else
                Return "传输数据为空！"
            End If
            result = req.GetResponse()
            ReceiveStream = result.GetResponseStream()
            sr = New StreamReader(ReceiveStream, encode)            Debug.WriteLine("已接收到响应流")            cProcessData = ""
            cProcessData = sr.ReadToEnd()            Debug.WriteLine("接收到的完整数据为：")
            Debug.WriteLine(cProcessData)
            Return cProcessData        Catch ex As Exception
            Return "找不到请求 URI 或者它的格式不正确"
        Finally
            If Not result Is Nothing Then
                result.Close()
            End If
        End Try    End Function