basic k way merge: terminate called after throwing an instance of 'std::logic_error' - algorithm

Question

I am trying to implement basic merge k sorted array algorithm but with strings.
I am getting the following error.
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_M_construct null not valid
The code works fine if the names vector has just 2 sub vectors, but when I add another list in the names vector I am getting the above error.
Whats wrong with the code?
#include <iostream>
#include <string>
#include <vector>
using namespace std;
vector<string> merge_2_names(vector<string> a, vector<string> b){
int i = 0 ; int j = 0;
vector<string> res;
while(i < a.size() && j < b.size()){
if (a[i].compare(b[j]) < 0){
res.push_back(a[i]);
i++;
}else{
res.push_back(b[j]);
j++;
}
}
while (i < a.size()){
res.push_back(a[i]);
i++;
}
while(j < a.size()){
res.push_back(b[j]);
j++;
}
return res;
}
vector<string> merge_k_names(vector<vector<string>> names){
vector<string> result;
cout << names.size() << "\n";
for (string s: names[0]){
result.push_back(s);
}
for(int i=1;i<names.size();i++)
{
result=merge_2_names(result,names[i]);
}
return result;
}
int main() {
vector<vector<string>> names {{"adam" , "raja" , "zync"},
{"edam" , "some" , "zian"},
{"mike" , "jimm" , "pame"}};
cout << names.size() << "\n";
vector<string> res = merge_k_names(names);
for (string s: res){
cout << s << " ";
}
return 0;
}