libxml不返回XPATH结果

我从相机下载了XML到内存，并想用libxml和Xpath对其进行解析，但是当我得到一个空白的结果(或者是" Leerer knoten"路径(，当试图查询节点的值。那里发生了什么？

std::string wrapper::utility::XMLHelper::getPayload(std::string identifier, std::string document)
{
    xmlDocPtr doc = xmlParseDoc((const unsigned char*)document.c_str());
    xmlXPathContextPtr xpathCtx;
    xmlXPathObjectPtr result;
    xpathCtx = xmlXPathNewContext(doc);
    if (xpathCtx == NULL)
        return "Invalid Context";
    result = xmlXPathEvalExpression((xmlChar*)identifier.c_str(), xpathCtx);
    char buffer[16];
    snprintf(buffer, sizeof(buffer), "%d", result->nodesetval->nodeNr);
    if (xmlXPathNodeSetIsEmpty(result->nodesetval))
    {
        xmlXPathFreeContext(xpathCtx);
        xmlXPathFreeObject(result);
        xmlFreeDoc(doc);
        return "Leerer Knoten";
    }
    std::string returnValue((char*)xmlNodeGetContent(result->nodesetval->nodeTab[0]));
    xmlXPathFreeContext(xpathCtx);
    xmlFreeDoc(doc);
    xmlCleanupParser();
    return returnValue;
}
std::string identifier = "/NetworkInterface/IPAddress/ipAddress";
std::string request =
    "<?xml version="1.0" encoding="UTF-8" ?> n"
    "<NetworkInterface version="2.0" xmlns="http://www.std-cgi.com/ver20/XMLSchema">n"
    "    <id>1</id> n"
    "    <IPAddress version="2.0" xmlns="http://www.std-cgi.com/ver20/XMLSchema">n"
    "       <ipVersion>dual</ipVersion> n"
    "       <addressingType>static</addressingType> n"
    "       <ipAddress>172.19.50.232</ipAddress> n"
    "       <subnetMask>255.255.0.0</subnetMask> n"
    "       <ipv6Address>::</ipv6Address> n"
    "       <bitMask>0</bitMask> n"
    "       <DefaultGateway>n"
    "           <ipAddress>172.19.50.1</ipAddress> n"
    "           <ipv6Address>::</ipv6Address> n"
    "       </DefaultGateway>n"
    "       <PrimaryDNS>n"
    "           <ipAddress>8.8.8.8</ipAddress> n"
    "       </PrimaryDNS>n"
    "       <SecondaryDNS>n"
    "           <ipAddress>0.0.0.0</ipAddress> n"
    "       </SecondaryDNS>n"
    "    </IPAddress>n"
    "    <Discovery version="2.0" xmlns="http://www.std-cgi.com/ver20/XMLSchema">n"
    "        <UPnP>n"
    "            <enabled>true</enabled> n"
    "        </UPnP>n"
    "        <Zeroconf>n"
    "            <enabled>true</enabled> n"
    "        </Zeroconf>n"
    "    </Discovery>n"
    "    <Link version="2.0" xmlns="http://www.std-cgi.com/ver20/XMLSchema">n"
    "        <MACAddress>c4:2f:90:4c:85:68</MACAddress> n"
    "        <autoNegotiation>true</autoNegotiation> n"
    "        <speed>10</speed> n"
    "        <duplex>half</duplex> n"
    "        <MTU>1500</MTU> n"
    "    </Link>n"
    "</NetworkInterface>";
std::string res = wrapper::utility::XMLHelper::getPayload(identifier, request);