Contact API ignores <= operator for custom date fields

Suppose you have a custom datetime field on contacts, and you want to search all contacts that have a date in this field that is earlier than some given date, like e.g.

    $result = civicrm_api3('Contact', 'get', array(
      'sequential' => 1,
      'custom_2' => array('<=' => "2015-08-31"),
    ));

the API ignores 'custom_2' => array('<=' => "2015-08-31") and returns all contacts instead.