Mongodb connection error after deploy to heroku

The app is working fine in local but after deploying to Heroku I am getting this error in heroku logs:

The `uri` parameter to `openUri()` must be a string, got "undefined". Make sure the first parameter to `mongoose.connect()` or `mongoose.createConnection()` is a string.

I have installed config, dotenv packages but still shows error in deployment.

Code:

require('dotenv').config();const connectDB = async () => {  try {    await mongoose.connect(process.env.MONGOURI, {      useNewUrlParser: true,      useUnifiedTopology: true,      useCreateIndex: true,      useFindAndModify: false,    });    console.log('MongoDB connected...');  } catch (err) {    console.log(err.message);    //Exit process with failure    process.exit();  }};connectDB();

env file:

MONGOURI = mongodb+srv://user:user@the-app.avsfx.mongodb.net/<myapp>?retryWrites=true&w=majority