Boost xpressive error

Question

This simple example using Boost xpressive (latest version) throws the error: Access violation reading location 0x000000. The error occurs on regex_match. This has to be something simple but I've looked at it until I am crazy.

Why does this not work?

using namespace boost::xpressive;

std::string hello( "Apple Orange_" );

const boost::xpressive::sregex rex = boost::xpressive::sregex::compile("(\\w+)\\s(\\w+)_"); 

boost::xpressive::smatch what;

if( regex_match( hello, what, rex ) )
{
  std::string s1 = what[0]; // whole match
  std::string s2 = what[1]; // first capture
}