mongodb query for first array element being "null" does not obey the usual "undefined properties match null" rule

gistfile1.txt

// This will find "bar" and "woo". But it does not find "foo".

(function() {
  db.dropDatabase();
  db.docs.insert([{
    title: 'foo',
    ids: []
  }, {
    title: 'bar'
  }, {
    title: 'baz',
    ids: [ 5 ]
  }, {
    title: 'woo',
    ids: [ null ]
  }]);
})();

data(db.docs.find({ 'ids.0': null }).toArray());


function data(o) {
  print(JSON.stringify(o, null, '  ') + '\n');
}