TypeError: Property 'source' of object #<Source> is not a function

[alex35mil]

Webpack 1.7.3 — ok.

Webpack 1.8.0:

/node_modules/webpack/lib/Compiler.js:253
                var content = source.source();
                                     ^
TypeError: Property 'source' of object #<Source> is not a function
    at Tapable.writeOut (/node_modules/webpack/lib/Compiler.js:253:26)
    at Tapable.<anonymous> (/node_modules/webpack/lib/Compiler.js:244:20)
    at /node_modules/webpack/node_modules/async/lib/async.js:125:13
    at Array.forEach (native)
    at _each (/node_modules/webpack/node_modules/async/lib/async.js:46:24)
    at Object.async.each (/node_modules/webpack/node_modules/async/lib/async.js:124:9)
    at Tapable.emitFiles (/node_modules/webpack/lib/Compiler.js:233:20)
    at /node_modules/webpack/node_modules/mkdirp/index.js:29:20
    at Object.oncomplete (fs.js:107:15)

Here is my plugin config:

new gzip({
  asset: '{file}.gz',
  algorithm: 'gzip',
  regExp: /\.js$|\.css$/
}),