Firebase Functions zwraca błąd “Error: Unexpected end of form at Multipart._final”

I started using Firebase Functions and ran into a problem. Namely, when I created my API (running locally), deployed to Firebase Functions using firebase deploy --only functions and called from my frontend – in the function logs an error is displayed:

Error: Unexpected end of form
at Multipart._final (/workspace/node_modules/busboy/lib/types/multipart.js:588:17)
at callFinal (node:internal/streams/writable:696:27)
at prefinish (node:internal/streams/writable:725:7)
at finishMaybe (node:internal/streams/writable:735:5)
at Multipart.Writable.end (node:internal/streams/writable:633:5)
at onend (node:internal/streams/readable:693:10)
at processTicksAndRejections (node:internal/process/task_queues:78:11)

My goal is to receive the file in the API and process it using the easy-template-x bibiotech. When I used multer for this, the problem was the same

index.js

const functions = require('firebase-functions');
const routes = require('./src/routes/routes');
const express = require('express');
const cors = require('cors');
const app = express();
app.use(cors());
app.use(express.json());

app.use('/', routes);

app.listen(3009, () => {
    console.log('Listening on 3001...');
});

exports.app = functions.https.onRequest(app);

routes.js

const checkErrors = require('../controllers/checkErrors');
const express = require('express');
const router = express.Router();

router.post('/checkErrors', checkErrors);
module.exports = router;

And checkErrors.js

const {TemplateHandler} = require('easy-template-x');
const handler = new TemplateHandler();
const busboy = require('busboy');

const checkErrors = async (req, res) => {
    const bb = busboy({headers: req.headers});
    let templateBuffer = null;
    let jsonData = '';

    bb.on('file', (fieldname, file, filename, encoding, mimetype) => {
        file.on('data', (data) => {
            templateBuffer = data;
        });
    });

    bb.on('field', (fieldname, val) => {
        jsonData += val;
    });

    bb.on('finish', async () => {
        jsonData = JSON.parse(jsonData);
        console.log(jsonData);
        const templateTags = await handler.parseTags(templateBuffer);
        const templateTagsNames = templateTags.map(({name}) => name);

        //Znajdź brakujące wartości w Excel i je zidentyfikuj
        const missingValues = [];
        jsonData
            .filter((row) => {
                return !templateTagsNames.every((name) => Object.keys(row).includes(name));
            })
            .forEach((row) => {
                templateTagsNames.forEach((templateName) => {
                    if (!row.hasOwnProperty(templateName)) {
                        missingValues.push({fieldName: templateName, rowNum: row.rowNum + 1});
                    }
                });
            });

        res.json({
            errors: missingValues,
        });
    });

    return req.pipe(bb);
};

module.exports = checkErrors;