XDocument.Descendents is not case-sensitive

I thought XML was case-sensitive? I am looking at XML files to find a field which will either be < Header > or < header >. I use the following code:

If Not xmlDoc.Descendants("Header") Is Nothing Then
    do something
ElseIf Not xmlDoc.Descendants("header") Is Nothing Then
    do something else
Else
    Print(1, "No header information found" & vbCrLf)
    messageText.Text = "Validation Complete"
    Return false
End If

So I am looking at an XML file that has < header > and the 'do something' line is being run! How can I make this a case-sensitive thingymabob?