[boost] Unescaped special character in argument <cxx, flags>/std:c++nn

When compiling with vc++ and specifying in my user_config.jam file: using msvc : 14.1c14 : : <cxxflags>/std:c++14 ; using msvc : 14.1c17 : : <cxxflags>/std:c++latest ; I am getting the messages: user-config.jam:5: Unescaped special character in argument <cxxflags>/std:c++14 user-config.jam:6: Unescaped special character in argument <cxxflags>/std:c++latest I am using the latest 'develop' branch of Boost Build. What is the workaround for this ?