NuSoap Webservice出错

Question

------------------------------- Web service asking for this information--------------------------------

<s:element name="PREPO0001">
<s:complexType>
<s:sequence>
<s:element minOccurs="0" maxOccurs="1" name="p1" type="s:string"/>
<s:element minOccurs="0" maxOccurs="1" name="p2" type="s:string"/>
</s:sequence>
</s:complexType>
</s:element>

------------------------------- Sending through Webservice Codeigniter--------------------------------

$this->nusoap_client = new nusoap_client("web-service link",'wsdl');
webservice_request= array('p1'=> $orden, 'p2' => $placa);
$row = $this->nusoap_client->call('PREPO0001',$webservice_request);

Getting an "ERROR" message from the WS any idea?

Erro from WS: {"PREPO0001Result":"ERROR"}