[boost] boost.format(): Unexpected assert failure?

The following snippet causes an assertion in debug mode on MSVC++ 8. ---cut--- string Output; string Blank; format fmt("Blank string: %s"); fmt % Blank; // This line causes an assertion in MSVC8 Output = fmt.str(); ---cut--- Rather than cause an assertion, shouldn't the formatter properly deal with the case of a zero-length string? Or is there a preferred way to handle this and/or parameter to pass to the formatter to handle zero-length strings?