MSXML4 And SelectSingleNode

The following is a code snippet of a classic ASP webapplication:

Set l_oNode = p_oXML.selectSingleNode(l_sXPath)
If (Not IsNull(l_oNode)) Then
l_sValue = l_oNode.text
End If

the selectSingleNode method should return Null when no node was found for the XPath expression, but it doesn't. At least not in a way that satisfies the "Not IsNull()" test. The program always tries to execute l_sValue = l_oNode.text which obviously results in errors when the node was not found.