(simple) boost thread_group question

I'm trying to write a fairly simple threaded application, but am new to boost's thread library. A simple test program I'm working on is:

#include <iostream>
#include <boost/thread.hpp>

int result = 0;
boost::mutex result_mutex;

boost::thread_group g;

void threaded_function(int i)
{
    for(; i < 100000; ++i) {}

    {
        boost::mutex::scoped_lock lock(result_mutex);
        result += i;
    }
}

int main(int argc, char* argv[])
{
    using namespace std;

    // launch three threads
    boost::thread t1(threaded_function, 10);
    boost::thread t2(threaded_function, 10);
    boost::thread t3(threaded_function, 10);

    g.add_thread(&t1);
    g.add_thread(&t2);
    g.add_thread(&t3);

    // wait for them
    g.join_all();

    cout << result << endl;

    return 0;
}

However, when I compile and run this program I get an output of

$ ./test 
300000
test: pthread_mutex_lock.c:87: __pthread_mutex_lock: Assertion `mutex->__data.__owner == 0' failed.
Aborted

Obviously, the result is correct but I'm worried about this error message, especially because the real program, which has essentially the same structure, is getting stuck at the join_all() point. Can someone explain to me what is happening? Is there a better way to do this, i.e. launch a number of threads, store them in a external container, and then wait for them all to complete before continuing the program?

Thanks for your help.