path::join() is broken

In file included from ./stout/include/stout/os.hpp:63,
                 from ./stout/include/stout/flags/flags.hpp:30,
                 from ./stout/include/stout/flags.hpp:17,
                 from stout/tests/flags_tests.cpp:7:
./stout/include/stout/path.hpp: In function ‘std::string path::join(const std::string&, T&& ...)’:
./stout/include/stout/path.hpp:42: error: ‘const struct std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ has no member named ‘back’
make[7]: *** [stout_tests-flags_tests.o] Error 1