I'm submitting a bug report
i have

webpackConfig.watchOptions = {
  poll: process.env.FORCE_POLL
}

in webpack config
WSL on Windows 10 does not implement fd, so no inotify for me, I have to use polling

the error I get is

- configuration.watchOptions.poll should be one of these:
  boolean | {
    "description": "`number`: use polling with specified interval.",
    "type": "number"
  }

console.log(process.env.FORCE_POLL) resolves to true

Webpack version:
2.1.0-beta.25

Please tell us about your environment:
WSL on Windows 10