riak mapreduce problems

mr.py

query = riakclient.add('products')
query.map("""
function(v, arg) { 
	var data = JSON.parse(v.values[0].data); 
	if(data.category == arg.category) { 
		return [[v.key, arg.category]]; 
	} else {
		return []; 
	}
}""", {"category":"food"})
for result in query.run():
	print result

out.py

[u'772216be03d211e2b7ae080027feaaaf', None]
[u'e90186a803d111e2b7ae080027feaaaf', None]
[u'ac8ddb0003d011e2b7ae080027feaaaf', None]
[u'132d425603d111e2b7ae080027feaaaf', None]