SOAP-ERROR: Parsing WSDL: Entity 'copy' not defined Magento

SOAP-ERROR: Parsing WSDL: Entity 'copy' not defined Magento

I try to use magento webservice but whene i try to call the webservice :
$proxy = new
SoapClient('http://mydomain.com/magento/index.php/api/v2_soap/?wsdl');
I get this error : Uncaught SoapFault exception: [WSDL] SOAP-ERROR:
Parsing WSDL: Couldn't load from
'http://mydomain.com/magento/index.php/api/v2_soap/?wsdl' : Entity 'copy'
not defined\n in /var/www/trunk/test.php:3\nStack trace:\n#0
/var/www/trunk/test.php(3),
So this Entity 'copy' not defined can you tell me why this exactly ,
should i add something in my server ?