Invalid options object. Dev Server has been initialized using an options object that does not match the API schema

I have been stuck on this error in my project when I add "proxy": "http://localhost:6000" in my package.json.

This is the error response after yarn start.

Invalid options object. Dev Server has been initialized using an options object that does not match the API schema.

• options.allowedHosts[0] should be a non-empty string. error Command failed with exit code 1. info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

But everything is fine when I remove the "proxy": "http://localhost:6000".

This is on my package.json:

{
  "name": "client",
  "version": "0.1.0",
  "private": true, 
  "dependencies": {
    "@material-ui/core": "^4.12.3",
    "@testing-library/jest-dom": "^5.16.1",
    "@testing-library/react": "^12.1.2",
    "@testing-library/user-event": "^13.5.0",
    "axios": "^0.24.0",
    "moment": "^2.29.1",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "react-file-base64": "^1.0.3",
    "react-redux": "^7.2.6",
    "react-scripts": "5.0.0",
    "redux": "^4.1.2",
    "redux-thunk": "^2.4.1",
    "web-vitals": "^2.1.2"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": [
      "react-app",
      "react-app/jest"
    ]
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  },
  "proxy": "http://localhost:6000"
}

Here is a workaround. Delete "proxy": "http://localhost:6000". Install package http-proxy-middleware with command npm install http-proxy-middleware --save. Create a file setupProxy.js inside your src folder or the root of your folder. Add these lines inside:

const { createProxyMiddleware } = require('http-proxy-middleware');

module.exports = function(app) {
  app.use(
    '/api',
    createProxyMiddleware({
      target: 'http://localhost:6000',
      changeOrigin: true,
    })
  );
};

Now, run your app. It should work.