WWW is not ready downloading yet?

import UnityEngine

class testXML(MonoBehaviour): 

def Start():
    Helper()

def Helper():       
    xmlURL = "http://files.unity3d.com/tom/orbitsim/XML/SolarSystem.xml"
    tWWW as WWW = WWW(xmlURL)
    yield tWWW
    guiText.text = tWWW.text

My guess is it's not downloading the XML file. I get the error "WWW is not ready downloading yet" if I comment out "yield tWWW", otherwise it just hangs. I am trying to parse an XML file in Boo, but I can't even get past the first part. Any ideas?

Thanks

Because yield requires a coroutine to work.

import UnityEngine

class testXML(MonoBehaviour): 

def Start():
    StartCoroutine(Helper());

def Helper():       
    xmlURL = "http://files.unity3d.com/tom/orbitsim/XML/SolarSystem.xml"
    tWWW as WWW = WWW(xmlURL)
    yield tWWW
    guiText.text = tWWW.text

Props on using Boo =)