Cannot set format of console log in Boost.Log

I'm trying to setup boost.log into a project I have. I've followed the examples, but when I try to set a format, the compiler says that format is a read-only reference.

void init() {
    logging::add_console_log(
        std::cout,

        // Compiler says "const boost::parameter::keyword<keywords::tag::format>& is read-only reference".
        keywords::format = "%Timestamp% %Message%",

        keywords::severity = info
    );

    logging::add_common_attributes();
}