Webpack Entry Descriptor Invalid Configuration Syntax

So i'm trying to configure my webpack config to use the entry descriptor syntax as described here.

This is an example of my config:

    entry: {
      shared: [
        'react',
        'prop-types',
        'lodash',
        'moment',
        './src/utils/polyfills.js',
    ],
    index: { import: './src/index.js', dependOn: 'shared' },
  }

However, when I run my build I'm constantly seeing errors saying this syntax doesn't match their API.

Start Command: "dev": "webpack --watch --config webpack.client.js"

Error

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration.entry should be one of these:
   function | object { <key>: non-empty string | [non-empty string] } | non-empty string | [non-empty string]
   -> The entry point(s) of the compilation.
   Details:
    * configuration.entry['index'] should be a string.
      -> The string is resolved to a module which is loaded upon startup.
    * configuration.entry['index'] should be an array:
      [non-empty string]
      -> A non-empty array of non-empty strings
    * configuration.entry['index'] should be one of these:
      [non-empty string]
      -> All modules are loaded upon startup. The last one is exported.
    * configuration.entry['index'] should be one of these:
      non-empty string | [non-empty string]
      -> An entry point with name

Has anyone seen this before? Any ideas?