更改 innerHTML 时提示返回未定义

所有的善良与和平！你知道有什么问题吗？editPrompt 函数在调用时返回 undefined。

<table class="table">
    <tr>
        <th>Имя</th>
        <th>Фамилия</th>
    </tr>
    <tr>
        <td>Имя</td>
        <td>Фамилия</td>
    </tr>
    <tr>
        <td>Имя</td>
        <td>Фамилия</td>
    </tr>
</table>

<form action="" id="form">
    <input type="text" name="" class="name" placeholder="Фамилия">
    <input type="text" name="" class="surname" placeholder="Имя">
    <button type="submit" class="btn">Добавить строку</button>
</form>

var submit = document.querySelector('.btn');
var inputName = document.querySelector('.name');
var inputSurname = document.querySelector('.surname');
var table = document.querySelector('.table');

submit.addEventListener('click', addTr);
function addTr() {
    var tr = document.createElement('tr');
    var tdName = document.createElement('td');
    var tdSurname = document.createElement('td');
    table.appendChild(tr);
    tr.appendChild(tdName);
    tr.appendChild(tdSurname);
    tdName.innerHTML = inputName.value;
    tdSurname.innerHTML = inputSurname.value;
}

table.addEventListener('click', editPrompt);

function editPrompt(event) {
    var editInnerHTML = prompt('Введите значение', event.target.innerHTML);
    event.target.innerHTML = editInnerHTML.value;
}