Heroku Builds succeeded but I am getting this Heroku logs error "MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined"

I am getting a application error screen, when I check the heroku logs for my app I get this error : MongooseError: 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.

in my App.js : require('./config/database'); my database.js

    const mongoose = require('mongoose');
const db = mongoose.connection;

mongoose.connect(process.env.DATABASE_URI, {
    useNewUrlParser: true,
});

db.on('connected', () => {
    console.log(`Connected to MongoDB on ${db.host}:${db.port}`);
});

in my .env files(changed credentials) : DATABASE_URI="mongodb+srv://admin:abc@cluster0.9vfr4.mongodb.net/statezz?retryWrites=true&w=majority"

Appreciate your help figuring out why is this happening ? the local version works just fine on my machine, and deployment was smooth but I am just getting the application error screen with no codes even!

thanks in advance.