Dear boost users, could someone tell me why this short program does not compile?

#include <boost/array.hpp>
#include <boost/static_assert.hpp>

int main(int argc, const char *argv[])
{
  boost::array<int, 3> a1 = {{1, 1, 1}};
  BOOST_STATIC_ASSERT(a1.static_size == 3);
  return 0;
}