express – proxy requests from localhost econnreset

Proxy error: Could not proxy request /api/customers from localhost:3000 to localhost:5000/.

See Here for more information (ECONNRESET).

fetch from react

fetch(`${process.env.REACT_APP_API_URL}/customers`)
    .then(res => res.json())
    .then(res => setData(res))
    .catch(err => console.error(err))

.env

REACT_APP_API_URL=/api

package.json

{
  "name": "client",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "dotenv": "^14.2.0",
    "react": "^16.2.0",
    "react-dom": "^16.2.0",
    "react-scripts": "1.0.17"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject"
  },
  "proxy": "http://localhost:5000"
}

server.js

const express = require('express');
const cors = require('cors');
const customersAPI = require("./server/customers/customers");
const bodyParser = require("body-parser");
const app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(cors());
app.use('/api/customers', customersAPI);

const port = 5000;

app.listen(port, () => `Server running on port ${port}`);