WebService with PHP + SOAP - Doubt with the return

I'm using a webservice to perform file uploads from the CAT52 gateway

WebService : link

You are doing all normal communication (at least I think) but there is a return that I can not understand

Return through vardump

  

object (stdClass) [9] public 'SendResult' = > string '999 | String   reference not set to an instance of a String. Parameter name: s'   (length = 74)

By the message is string that was not instantiated, parameter 's', however I do not have this parameter.

Follow the code so you can help me

    $headerBody = array('CNPJ' => $this->getCnpj(),
                            'CategoriaUsuario' => $this->getCategoriaUsuario(),
                            'Senha' => $this->getSenha(),
                            'Usuario' => $this->getUsuario());


    $header = new SOAPHeader($this->getXmlns(), 'Autenticacao', $headerBody);

    $this->cliente = new SoapClient( $this->getWsdlUrl(), array('soap_version' => SOAP_1_2));
    $this->cliente->__setSOAPHeaders($header);

$filename = new SoapParam($arquivoNome, 'NomeArquivo');
        $content = new SoapParam(utf8_encode($arquivoConteudo), 'ConteudoArquivo');
        $tipoenvio = new SoapParam($envioNormal, 'EnvioNormal');
        $obs = new SoapParam($observacoes, 'Observacoes');

        //$resEnvio = $this->cliente->__soapCall('Enviar', $token);
        $resEnvio = $this->cliente->Enviar($filename, $content, $tipoenvio, $obs);
        var_dump($resEnvio);