Overloaded ostream operator segmentation fault if no endl

Question

class foo {
    public:
    friend ostream& operator << (ostream &os, const foo &f);
    foo(int n) : a(n) {}
    private:
    vector <int> a;
};

ostream& operator << (ostream &os, const foo &f) {
    for (int i = 0; i < f.a.size(); ++i)
        os << f.a[i] << " ";
    os << endl; // why is this line a must?
}

int main(void) {
    foo f(2);
    cout << f << endl;
    return 0;
}

In the above code, if the marked line is removed, there will be a segment fault error, can someone explain why?