webpack.DefinePlugin and process.env.DEBUG

My webpack config:

...
plugins: [
  new webpack.DefinePlugin({
    'process.env': {
      NODE_ENV: JSON.stringify('production'),
      DEBUG: true
    }
  })
],

target: 'electron-main',
...

So webpack changes
process.env.DEBUG = namespaces;
to
true = namespaces;
which leading to error
ReferenceError: Invalid left-hand side in assignment