Hello, Here is a sample that generates an assertion failure with Visual C++ 2005. A NULL ptr is passed to std::string::append() by boost::io::detail::mk_str(). //////////////////////////////////////////////////////////////////// #include int main(int argc, char* argv[]) { std::string str(""); // Empty string. boost::format frmt( "%s" ); std::string res = ( frmt % str ).str(); return 0; } //////////////////////////////////////////////////////////////////// This is due to all the assert statements added in the new "secure" STL that is shipped with VC2005. But still, I don't think passing NULL to string::append() is correct anyway... Regards. J.S.Otis Software Developper InnovMetric Software Inc.