C++vector subscript out of range错误

Question

在vs2022中编辑了一小段代码。编译后运行，输入一个词后回车就提示vector subscript out of range。代码如下所示。请各位达人多多指教。应该做何修改。
word.h

#pragma once
#ifndef a_word
#define a_word
#include<vector>
#include<iostream>
#include<string>
using std::vector;
using std::istream;
using std::string;
using std::cin;
using std::cout;
using std::endl;
istream& word(istream&, vector<string>&, vector<int>&);
#endif

word.cpp

#include"word.h"
istream& word(istream& is, vector<string>& wd, vector<int>& sums) {
    string s;
    while (cin >> s) {
        bool flag = false;
        for (vector<string>::size_type i = 0; i <= wd.size(); ++i) {
            if (s == wd[i]) {
                flag = true;
                sums.push_back(i);
                break;
            }
        }
        if (flag==false) {
            wd.push_back(s);
            sums.push_back(1);
        }
    }
    return is;
}

main

#include"word.h"

int main() {
    vector<string> wd1;
    vector<int> sums1;
    word(cin, wd1, sums1);
    if (wd1.size() == 0) {
        cout << "向量为空。";
    }else
    {
        cout << "一共有" << wd1.size() << "个单词。" << endl;
    }
    
    for (vector<string>::size_type i = 0; i != wd1.size(); ++i) {
        cout << wd1[i] << "出现" << sums1[i] << "次" << endl;
    }
    return 0;
}

Answer 1

i <= wd.size()
改为
i < wd.size()