webpackExtraArgs is ignored

[doofin]

I tried to set devtool = false in webpack config,but passing webpackExtraArgs := Seq("--devtool", "false") in sbt doesn't work and gives warning

there's a key that's not used by any other settings/tasks:
[warn]  
[warn] *  webpackExtraArgs

but using js config file it works :

var webpack = require('webpack');
module.exports = require('./scalajs.webpack.config');
// And then modify `module.exports` to extend the configuration
module.exports.devtool = false

with "ch.epfl.scala" % "sbt-scalajs-bundler" % "0.21.1"