The controller json data in step 4 of the tutorial doesn't have 'age' attribs but the documentation claims that it should:

e.g. instead of:
{'name': 'Nexus S',
'snippet': 'Fast just got faster with Nexus S.'},

should have:
{'name': 'Nexus S',
'age': 1,
'snippet': 'Fast just got faster with Nexus S.'},

Note that this doesn't really cause any problems (other than confusion for the reader) since the implicit ordering in the array comes through, presumably because the null age attribs all sort the same and the implicit order ends up being expressed).