Hi, This code: boost::format("%1%") % ""; on Visual C++ 2005 with the following call stack: msvcp80d.dll!std::_Debug_message msvcp80d.dll!std::_Debug_pointer<char> msvcp80d.dll!std::basic_string<char,std::char_traits<cha r>,std::allocator<char> >::_Inside msvcp80d.dll!std::basic_string<char,std::char_traits<cha r>,std::allocator<char> >::append MyApp.exe!boost::io::detail::mk_str<char,std::char_trait s<char>,std::allocator<char> > MyApp.exe!boost::io::detail::put<char,std::char_traits<c har>,std::allocator<char>,char const (&)[1]> MyApp.exe!boost::io::detail::distribute<char,std::char_t raits<char>,std::allocator<char>,char const (&)[1]> MyApp.exe!boost::io::detail::feed<char,std::char_traits< char>,std::allocator<char>,char const (&)[1]> MyApp.exe!boost::basic_format<char,std::char_traits<char >,std::allocator<char> >::operator%<char const [1]> Produces the following assert message: invalid null pointer Because at > TheStockGuy.exe!boost::io::detail::mk_str<char,std::char _traits<char>,std::allocator<char> > it's called the std::basic_string<char,std::char_traits<cha r>,std::allocator<char> >::append function with arguments NULL and 0 [Basically that library makes this call: String.append(NULL, 0); wich is invalid by ISO C++.] Mystic Devil