Error with webpack

Done in 1.75s.

yarn run v0.21.3

$ yarn run webpack -- --config webpack.config.js 

yarn run v0.21.3

$ "/home/zangetsu/devel/tmp/pgadmin4/web/node_modules/.bin/webpack" --config webpack.config.js

module.js:491

    throw err;

    ^

Error: Cannot find module 'optimize-css-assets-webpack-plugin'

    at Function.Module._resolveFilename (module.js:489:15)

    at Function.Module._load (module.js:439:25)

    at Module.require (module.js:517:17)

    at require (internal/module.js:11:18)

    at Object.<anonymous> (/home/zangetsu/devel/tmp/pgadmin4/web/webpack.config.js:11:33)

    at Module._compile (module.js:573:30)

    at Object.Module._extensions..js (module.js:584:10)

    at Module.load (module.js:507:32)

    at tryModuleLoad (module.js:470:12)

    at Function.Module._load (module.js:462:3)

error Command failed with exit code 1.

info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

error Command failed with exit code 1.

info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

error Command failed with exit code 1.

info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

commit c4393a7795eef18b311724f45b6c9c0dae01926d (HEAD -> master, origin/master, origin/HEAD)

Author: Surinder Kumar <surinder.kumar@enterprisedb.com>

Date:   Thu Aug 31 14:50:22 2017 +0100