Searching XML using XBRL Schema

[mchac]

I'm trying to pull specific data from XML files using XBRL structure. An example XML file is here
Index of /Archives/edgar/data/104169/000010416912000018
file: 0000104169-12-000018-xbrl.zip
(it's a Walmart financial filing from 31oct2012)

My understanding is that once that file is downloaded it's possible to pull specific bits of data using VB. I know very little about XML parsing and search but two sources I've found suggest a simple sub similar to the following. However I'm getting Run-time error 91 and I can't undrstand why the object variable isn't set. Does anyone see an obvious error in this?

Sub GetXBRLData()
    Dim LOADFILE As String
    LOADFILE = "'path to file'\wmt-20121031.xml"   'xml instance document
    
    Dim CONCEPT As String
    CONCEPT = "us-gaap:AccountsPayableCurrent"
    
    Dim CONTEXT As String
    CONTEXT = "I2013Q3"
    
    Dim oDocument As MSXML2.DOMDocument30
    Set oDocument = New MSXML2.DOMDocument30
    
    oDocument.async = False
    oDocument.validateOnParse = False
    oDocument.Load (LOADFILE)
    
    Dim FACTVALUE As String
    FACTVALUE = oDocument.SelectSingleNode("/" & CONCEPT & "[@contextref='" & CONTEXT & "']").Text
    MsgBox FACTVALUE
    
    Set oDocument = Nothing
End Sub

 

[mchac]

In case anyone is interested in getting this kind of data, my error was in the FACTVALUE line. Needs "//" not "/" and "contextRef" not "contextref".

FACTVALUE = oDocument.SelectSingleNode("//" & CONCEPT & "[@contextRef='" & CONTEXT & "']").Text