c++ std::thread segmentation fault

I am trying to use threads to increase my work throughput, however, I am coming up with some weird errors that I don't know the cause of. The following is not my code, but is the minimal code that reproduces the error:

main.cpp

#include<iostream>
#include<thread>
#include<functional>
#include<vector>
using namespace std;

const int THREADCOUNT = 2;

void Join(vector<thread>& T);
void Fac(int num, int& answer);

template<typename T>
ostream operator<<(ostream& os, const vector<T>& input)
{
    for (int i = 0; i < input.size(); i++)
    {
        os << input[i] << '\t';
    }
}

int main (int argc, char * argv[])
{
    if (argc != 3) return 1;

    int start = atoi(argv[1]);
    int  end  = atoi(argv[2]);

    vector<thread> parallel(THREADCOUNT);
    vector<int> answers(end - start + 1);

    for (int n = start; n <= end; n++)
    {
        if (parallel[n % THREADCOUNT].joinable())
        {
            parallel[n % THREADCOUNT].join();
        }

        parallel[n % THREADCOUNT] = thread(Fac, n, ref(answers[n - start]));
    } //n
    Join(parallel);
    cout << answers.size() << endl;
    cout << answers << endl;
    return 0;
} //end main

void Join(vector<thread>& T)
{
    for (auto& thread : T)
    {
        if (thread.joinable()) thread.join();
    }
}

void Fac(int num, int& answer)
{
    if (num < 1) { answer = 1; return; }
    answer = 1;
    while (num != 1)
    {
        answer = num * answer;
        num--;
    }
}

The segmentation fault occurs when I try to retrieve the data inside the "answers" std::vector. I am on a CentOS Linux 7, and compiling with this command:

g++ -std=c++11 main.cpp -o T.exe -lpthread

What exactly is going on? I stared at it for a while and looked at some pages on StackOverflow but I can't seem to understand what's going on?