[boost] Question about shared_prt doc's

In the documentation for the shared_ptr constructor that takes a weak_ptr parameter, it says that if the weak_ptr is empty then so is the constructed shared_ptr. But it also says that if the weak_ptr has a use_count() of 0 then a bad_weak_ptr exception is thrown. Since an empty weak_ptr has a use_count() of 0, these statements can't both be true. Which one is? Joe Gottman