c++ map<string,int> 用下标访问的时候报错的问题？

Question

#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<algorithm>
using namespace std;

map<string,int> m;
vector<int> v;
int index=1;

int main(){

    int n,k;
    cin>>n>>k;
    for(int i=0;i<k;i++){
        int course,m;
        cin>>course>>m;
        for(int j=0;j<m;j++){
            string s;
            cin>>s;
            if(m[s]!=0){
                v[m[s]].push_back(course);
            }else {
                m[s]=index;
                index++;
                v[m[s]].push_back(course);
            }
        }
    }
} 

报错
22 10 D:\Dev C++\workspace\A1039.cpp [Error] no match for 'operator[]' in 'm[s]'

每一次用下标访问m[s]的时候都会报错