Seg fault in unordered_map

I am observing segmentation fault in the following snippet of code:

void read_mdl(const std::string& fname) {
        std::ifstream f(fname);

        std::unordered_map<int, std::vector<double>> pt_;

        while(f) {
            int xi = -1;
            int pa = 0;
            double score = -1;
            f >> xi >> pa >> score;
            if(xi == -1) { break; }

            std::unordered_map<int, std::vector<double>>::iterator it = pt_.find(pa);

            std::cout << xi << " " << pa << " " << score << std::endl;

            if (it != pt_.end()) {
                pt_.insert(std::make_pair(pa, std::vector<double>(n_, 0)));
            }

            (it->second)[xi] = score; // causing segmentation fault
        }
    }

Error occurs while accessing the value associated with the key in the unordered map. Any pointers as where I am making the mistake?