WebClient generates exception: header must be modified using ...

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The following code generates the exception shown below when I call
UploadData. I'm trying to call a WCF service using basicHTTP with a manually
generated Soap message. Am I on the right track with this code? Thanks!!

Code:

Dim recvDoc As New Xml.XmlDocument
recvDoc.LoadXml("<s:Envelope... Soap XML goes here")

Dim wc As New System.Net.WebClient()
With wc
Dim str() As Byte =
System.Text.Encoding.UTF8.GetBytes(recvDoc.OuterXm l)

.Encoding = System.Text.Encoding.UTF8
.Headers.Add(Net.HttpRequestHeader.ContentType, "text/xml")
.Headers.Add(Net.HttpRequestHeader.ContentLength,
str.GetLength(0).ToString)
.Headers.Add(Net.HttpRequestHeader.Accept, "*/*")
.Headers.Add(Net.HttpRequestHeader.KeepAlive, "1")

Dim respStr As String =
System.Text.Encoding.UTF8.GetString(wc.UploadData( "http://localhost/DevWcf/WcfSimpleTest/WcfSimpleTest.svc", str))

Dim respDoc As New Xml.XmlDocument
respDoc.LoadXml(respStr)
End With

Exception:
"This header must be modified using the appropriate property. Parameter
name: name"