Issuing a POST request with a large amount of data fails in VB6

I have a pretty standard function to post some XML string data to a remote WCF service and extract the result. It works fine, but fails to scale to a "large" amount of data (138KB in this case.)

' performs a HTTP POST and returns the resulting message content
Function HttpPost(sUrl As String, sSOAPAction As String, sContent As String) As String
  Dim oHttp As Object
  'Set oHttp = CreateObject("Microsoft.XMLHTTP")
  Set oHttp = CreateObject("MSXML2.XMLHTTP.6.0")
  oHttp.open "POST", sUrl, False
  oHttp.setRequestHeader "SOAPAction", """http://conducive.com.au/IXpacManagement/" & sSOAPAction & """"
  oHttp.setRequestHeader "Content-Type", "text/xml; charset=utf-8"
  oHttp.setRequestHeader "Content-Length", Len(sContent)
  oHttp.send Str(sContent)

  If oHttp.status = 200 Then
    HttpPost = oHttp.responseText
  Else
    MsgBox "An error (" & LTrim(Str(oHttp.status)) & ") has occurred connecting to the server."
    HttpPost = ""
  End If

  Set oHttp = Nothing

End Function

When I use Microsoft.XMLHTTP I get error 7 out of memory.
When I use MSXML2.XMLHTTP.6.0 I get object doesn't support this property or method.
In either case sending a small string of under a thousand characters works perfectly.

Here's what I get when I try different ways of sending the string:

Using oHttp.send(sContent): Even small POSTs fail with Invalid procedure call or argument Runtime error 5.
Using oHttp.send sContent: Even small POSTs fail with Invalid procedure call or argument Runtime error 5.

In the end oHttp.send CStr(sContent) worked. Thank you all for the suggestions because I was lost.

Solution

Try taking out the Str() around sContent and just using parentheses to pass it by value, e.g.

oHttp.send (sContent)

or failing that at least use CStr() - Str() is supposed to convert numbers to strings.