2 sends

I'm using MSXML2.XMLHTTP to send XML data to an ASP page,
'*****************************************************
' This is a test on VB

   Dim doc As MSXML2.DOMDocument40
   Dim docRet As MSXML2.DOMDocument40
   Dim url As String

   Set doc = New MSXML2.DOMDocument40
   If Not doc.Load(xmlFile) Then
       Call Err.Raise(vbObjectError, "Here", "Load Error")
   End If

   Dim http As MSXML2.XMLHTTP40
   Set http = New MSXML2.XMLHTTP40

   Call http.open("POST", url, False)
   Call http.send(doc)
   Set docRet = http.responseXML

   MsgBox docRet.xml
'*****************************************************

it fails at "send" method,
but if I send an empty request just before...

   Call http.open("POST", url, False)
   Call http.send