terminate called after throwing an instance of 'std::logic_error' what(): basic_string::_M_construct null not valid

#include<bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int mx = 0;
  string mxs = 0;
  int c = 0;
  for (int i=1;i<=n;i++) {
      string name;
      int qc,bc,lun;
      char gan,xi;
      int fen;
      cin >> name >> qc >> bc >> gan >> xi >> lun;
      if (qc > 80 && lun >= 1) {
          fen += 8000;
      }
      if (qc > 85 && bc > 80) {
          fen += 4000;
      }
      if (qc > 90) {
          fen += 2000;
      }
      if (qc > 85 && xi == 'Y') {
          fen += 1000;
      }
      if (bc > 80 && gan == 'Y') {
          fen += 850;
      }
      c += fen;
      if (fen > mx) {
          mx = fen;
          mxs = name;
      }
  }
  cout << mxs << endl << mx << endl << c;
}