如何在magento api V1中检索客户订单

I am using magento API V1. I want to retrieve specific customer orders. I am using order.list method. In this method my filter is not working.It is giving me complete order list. I don't know where i am mistaking. Here is my code

$client = new SoapClient('http://magentohost/api/soap/?wsdl');

$session = $client->login('apiUser', 'apiKey');

$filter = array('filter' => array(array('key=' => 'customer_id', 'value' => 210)));

$result = $client->call($session, 'order.list',$filter);
var_dump ($result);