webservice result return null in flex4

Question

hi,

I have a web service on my localhost. Calling this from a PHP page works fine and it returns the expected results. The result is:

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope
xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:ns4="http://10.43.146.188/webService/"
SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"&gt;
<SOAP-ENV:Body>
<ns4:pingResponse>
<return xsi:type="xsd:string">pong</return>
</ns4:pingResponse>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

in Flex4:

var service:WebService = new WebService();
service.wsdl = address+"?wsdl";
service.loadWSDL();
service.ping.addEventListener(ResultEvent.RESULT,OnUploadResult);  
service.ping("test");        

private function OnUploadResult(event:ResultEvent ):void  
{  
     //event.result return is null
      Alert.show(event.result as String);  
} 

The "OnUploadResult" method is called, but event.result is null,

Can anyone see if I am missing something? is there something wrong?