c++ terminate std::logic error ......where did my code go wrong? - c++

Question

I'm encountering this really weird error and I dont know where I'm constructing a string with a null parameter, could someone please help me out? Thanks!
Here's the error:
terminate_called_after_throwing_an_instance_of_'std::logic_error'
__what():__basic_string::_M_construct_null_not_valid
Here's my code:
/*
ID: 2005amr1
TASK: friday
LANG: C++
*/
#include <iostream>
#include <fstream>
#include <string>
#include <map>
#include <vector>
using namespace std;
vector <string> days = {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"};
bool leap (int y){
if (y%100 == 0){
if (y%400 == 0){
return true;
}
else{
return false;
}
}
else if (y%4 == 0){
return true;
}
else{
return false;
}
}
int targetedSearch (vector<string> array, string key){
int index = 0;
while( (index < array.size( )) && (key != array[index])){
if (array[index] != key){
index++;
}
}
return (index);
}
string findLastDay(string FDAY, int daysInMonth){ // all good here
int indexOfString = targetedSearch(days, FDAY);
if (FDAY == "Mon" && daysInMonth == 28){
return ("Sun");
}
return (days[(daysInMonth - (7*(daysInMonth/7))-1) + indexOfString]);
}
string find13Day (string firstDay){ // all good here
int indexOfString = targetedSearch(days, firstDay);
return (days[indexOfString+5]);
}
string findFirstDayNextMonth (string LDAY){
int indexOfString = targetedSearch(days, LDAY);
if (indexOfString+1 > 6){
return (days[(indexOfString+1)-7]);
}
else{
return (days[indexOfString+1]);
}
}
int numDays (int month, int year){
switch(month) {
case 0:
return 31;
break;
case 1:
if (leap(year) == true){
return 29;
}
else{
return 28;
}
break;
case 2:
return 31;
break;
case 3:
return 30;
break;
case 4:
return 31;
break;
case 5:
return 30;
break;
case 6:
return 31;
break;
case 7:
return 31;
break;
case 8:
return 30;
break;
case 9:
return 31;
break;
case 10:
return 30;
break;
case 11:
return 31;
break;
}
return 31;
}
int main() {
ofstream fout ("friday.out");
ifstream fin ("friday.in");
string startDay = "Mon";
int dayValues[7] = {0,0,0,0,0,0,0};
int n;
fin >> n;
map <string,int> dayValue;
for (int i = 1900; i< 1900+n; i++){
for (int m =0; m<12; m++){
int PAN = numDays (m,i);
string tday = find13Day(startDay);
dayValues[targetedSearch(days,tday)]++;
string LDAY = findLastDay(startDay, PAN);
startDay = findFirstDayNextMonth(LDAY);
}
}
fout << " " << dayValues[5] << " " << dayValues[6]<< " " << dayValues[0]<< " " << dayValues[1]<< " " << dayValues[2]<< " " << dayValues[3]<< " " << dayValues[4];
}