The following code causes a runtime assertion in VC 8.0: std::string l_param; std::string l_str = (boost::format("here is an empty string: %1%") % l_param).str(); It passes an empty string argument to boost::format. The assert is inside the implementation of basic_string::append() and boils down to the fact that the implementation of boost::format ends up calling basic_string::append(const char *, size_type) with a null pointer, which causes basic_string::append to assert. The following sample code causes the same assertion without boost::format being involved: std::string l_str; const char *l_null = NULL; std::string::size_type l_size = 0; l_str.append(l_null, l_size); // causes an assert I have currently disabled the assertion by calling the following code before I do anything else: #if _MSC_VER >= 1400 _CrtSetReportMode( _CRT_ASSERT, _CRTDBG_MODE_FILE); _CrtSetReportFile( _CRT_ASSERT, _CRTDBG_FILE_STDERR ); #endif which causes the asserts to simply emit a message to stderr rather than to pop up a message box, but I don't really want to disable VC 8.0's assertions across the board. My questions are: 1) Is this considered to be a bug in boost::format or a case of VC 8.0's runtime checks being overzealous? Personally I feel that calling append(null, 0) should be allowed and VC 8.0 is being too protective, but the copy of the C++ standard I have suggests that VC 8.0 is correct. 2) Either way, what do people think is the best way forward? thanks, mick