Elastica geo search problem

<?php

// some geo locations
$brandenburgerTor = array(
  'lat' => 52.516455903398,
  'lon' => 13.380317687988
);

$friedrichstrasse = array(
  'lat' => 52.519772437396,
  'lon' => 13.388257026672
);

$alex = array(
  'lat' => 52.521548116471,
  'lon' => 13.410186767578
);

// setting up the client, index and type
$client = new Elastica_Client();

$index = $client->getIndex('test');

$index->create(array(), true);

$type = $index->getType('product');

// mapping
$type->setMapping(array(
  'product' => array(
    'properties' => array(
      'title'    => array('type' => 'string', 'store' => 'yes'),
      'location' => array('type' => 'geo_point', 'store' => 'yes')
    )
  )
));


// create two documents of two different locations

$data = array(
  'product' => array(
    'title' => 'Product A',
    'location' => $brandenburgerTor
  )
);

$docs = array();

$docs[] = new Elastica_Document('doc1', $data);

$data['product']['title'] = 'Product B';
$data['product']['location'] = $alex;

$docs[] = new Elastica_Document('doc2', $data);

$type->addDocuments($docs);


// do the query using a geo location in the near of the documents

$filter = new Elastica_Filter_GeoDistance('product.location', $friedrichstrasse['lat'], $friedrichstrasse['lon'], '100km');
$all = new Elastica_Query_MatchAll();
$query = new Elastica_Query_Filtered($all, $filter);


$results = $index->search($query);

foreach ($results as $result)
{
  echo $result->getId()."\n";
}

if (0 == count($results))
{
  echo "nothing found\n";
}

// problem is: nothing is found, but why?