### Describe the bug

https://github.com/developer-choi/axios-error-report

TypeError is occurred (in 1.6.6 version)

<img width="184" alt="image" src="https://github.com/axios/axios/assets/51902362/1f7f6e7f-8c8b-4ef7-9854-5c08f572d58b">

<img width="171" alt="image" src="https://github.com/axios/axios/assets/51902362/36042439-a071-485e-b696-9f2c01849759">

### To Reproduce

https://github.com/developer-choi/axios-error-report

### Code snippet

_No response_

### Expected behavior

_No response_

### Axios Version

1.6.6

### Adapter Version

_No response_

### Browser

CHROME

### Browser Version

120.0.6099.225

### Node.js Version

20.11.0

### OS

Window 11
test/unit/adapters/http.js
});
    describe('toFormData helper', function () {
      it('should properly serialize nested objects for parsing with multer.js (express.js)', function (done) {
        var app = express();
        var obj = {
          arr1: ['1', '2', '3'],
          arr2: ['1', ['2'], '3'],
          obj: {x: '1', y: {z: '1'}},
          users: [{name: 'Peter', surname: 'griffin'}, {name: 'Thomas', surname: 'Anderson'}]
        };
        app.post('/', multer().none(), function (req, res, next) {
          res.send(JSON.stringify(req.body));
        });
        server = app.listen(3001, function () {
          // m
          // a
          // a
          // -
          // a
          // a
          // -
          // a
          // a
          // -
          Promise.all([null, false, true].map(function (mode) {
            return axios.postForm('http://localhost:3001/', obj, {formSerializer: {indexes: mode}})
              .then(function (res) {
                assert.deepStrictEqual(res.data, obj, 'Index mode ' + mode);
              });
          })).then(function (){