Externals defined in webpack.config still getting error module not found

I have defined externals in webpack.config for material-ui

module.exports = [{
  entry: ...
  output:...
  externals: {
    react: {
      commonjs: "react",
      commonjs2: "react"
    },
    "material-ui": {
      commonjs: "material-ui",
      commonjs2: "material-ui"
    }
  },
  module: ...
}];

Still its giving error like-

Cannot resolve module 'material-ui/IconButton'......

In my entry js file, I have

import React, {Component} from "react";
import IconButton from "material-ui/IconButton";
.....
.....

Ok I solved it. External expects complete path.

So either,

import {IconButton} from "material-ui"

or

externals: {
  "material-ui/IconButton": {
    commonjs: "material-ui/IconButton",
    ...
  }
}

will work. Ofcourse, second option is not reasonable here