webpack.extractText = false is considered invalid

[zawupf]

This issue is a: Bug report

node v8.9.4
yarn 1.3.2
nwb v0.21.0

To disable ExtractWebpackPlugin the docs still recommend to set the corresponding option to false.
But when checking the config:

module.exports = {
  webpack: {
    extractText: false
  }
}

this error is reported:

× webpack.extractText = 'type: boolean'
  Must be an Object

Note: Using { disable: true } instead of false seems to be a workaround.