My web client error:Content-Length or Chunked Encoding cannot be set for an operation

[groads2]

I am trying to learn how to write a web client. I wrote a WCF web client to the same site in the code below and that worked. I would also like to write one similar to the code below. Any ideas on what I am doing wrong would be great. I am getting the error:

InnerException = {"Content-Length or Chunked Encoding cannot be set for an operation that does not write data."}

Public Class Form1
Dim manualWebClient As New System.Net.WebClient()
  Private Sub Jack() Handles Button1.Click
       manualWebClient.Headers.Add("Content-Type", "text/xml;  charset=utf-8")
       manualWebClient.BaseAddress = "http://ws.cdyne.com"
       Dim reqstr As String = "<s11:Envelope xmlns:s11=""http://schemas.xmlsoap.org/soap/envelope/"">" & System.Environment.NewLine & _
           "<s11:Body>" & System.Environment.NewLine & _
           "<ns1:GetCityForecastByZIP xmlns:ns1=""http://ws.cdyne.com/WeatherWS/"">" & System.Environment.NewLine & _
           "<ns1:ZIP>33569</ns1:ZIP>" & System.Environment.NewLine & _
           "</ns1:GetCityForecastByZIP>" & System.Environment.NewLine & _
           "</s11:Body>" & System.Environment.NewLine & _
           "</s11:Envelope>"
       ' manualWebClient.Headers.Add("Content-Length", Len(reqstr).ToString)
       Dim bytArguments As Byte() = System.Text.Encoding.ASCII.GetBytes(reqstr)
       Try
           Dim bytRetData As Byte() = manualWebClient.UploadData("/WeatherWS/Weather.asmx", "POST", bytArguments)
           MessageBox.Show(System.Text.Encoding.ASCII.GetString(bytRetData))
       Catch ex As Exception
           Dim v As String = ex.ToString
       End Try

   End Sub
End Class

Edited September 19, 2012 by PlausiblyDamp