Strange secondary sorting of Elastic Search TermsFacet entries with identical hit count

gistfile1.sh

curl -XDELETE 'http://localhost:9200/test/'
curl -XPUT 'http://localhost:9200/test/'

curl -XPUT 'http://localhost:9200/test/test/1' -d '
{
    "doc":{
        "string" : "foo"
    }
}
'

curl -XPUT 'http://localhost:9200/test/test/2' -d '
{
    "doc":{
        "string" : "bar"
    }
}
'

curl -XGET 'http://localhost:9200/test/test/_search?pretty=true' -d '
{
  "query" : {
    "match_all" : {
    }
  },
  "facets" : {
    "myFacet" : {
      "terms" : {
        "field" : "doc.string",
        "size" : 10,
        "order" : "count"
      }
    }
  }
}
'

#Response shows strange secondary sorting by term value
  ...
  "facets" : {
    "myFacet" : {
      "_type" : "terms",
      "missing" : 0,
      "terms" : [ {
        "term" : "foo",
        "count" : 1
      }, {
        "term" : "bar",
        "count" : 1
      } ]
    }
  }