Node.js 错误：enoint:没有这样的文件或目录，stat'/app/client/build/index.html'；

我的应用程序运行得非常好，但每当我尝试将其部署到Heroku时，它总是会出现此错误

index.js：

const express = require('express');
const mongoose = require('mongoose');
const cookieSession = require('cookie-session');
const passport = require('passport');
const bodyParser = require('body-parser');
const keys = require('./config/keys');
require('./models/User');
require('./models/Asset');
require('./models/Episode');
require('./config/passportConfig');

mongoose.connect(keys.mongoURI, { useNewUrlParser: true });

const app = express();

app.use(bodyParser.json());
app.use(
    cookieSession({
        maxAge: 30 * 24 * 60 * 60 * 1000,
        keys: [keys.cookieKey]
    })
);

app.use(passport.initialize());
app.use(passport.session());

require('./routes/authRoutes')(app);
require('./routes/assetsRoutes')(app);

if(process.env.NODE_ENV === 'production') {
    app.use(express.static('client/build'));

    const path = require('path');
    app.get('*', (req, res) => {
        const index = path.join(__dirname, 'client', 'build', 'index.html');
        res.sendFile(index);
    });
}

const PORT = process.env.PORT || 5000;
app.listen(PORT, () => {
    console.log(`server is up at port ${PORT}.`)
});

html确实存在，但Heroku似乎从未找到它

请忽略这只是一个填充，因为StackOverflow不会让我 发布一个没有细节的问题，但我没有太多细节要补充。 ：）