stringstream str() function retuns bad ptr

Question

I am trying to read a XML file and buffer it in a std::stringstream, but when I call str() function on the buffered data I am getting a bad ptr..

Code...

std::stringstream pushFileToStream(const char* xmlFile)
{
    std::stringstream buffer;

    std::ifstream file(xmlFile);
    if ( file )
    {
        buffer << file.rdbuf();
        file.close();
    }
    return buffer;
}

int main()
{
    const char* xmlFile = "resources/sample.xml";
    std::stringstream inputLicenseFile = pushFileToStream(xmlFile);

    std::string strXml = inputLicenseFile.str();  <--- bad ptr..
    return 0;
}