source map file almost empty when devtool property exists

We are using webpack version 1.x. I’m playing with source-maps file and how to use it in Chrome along with the minified/uglified javascript code. Below is my webpack config. I’m wondering why the contents of the map file is so small when devtools property exists.

Here is the contents of the map file when devtool property exists

{"version":3,"sources":[],"names":[],"mappings":"","file":"assets/js/app.bundle91fcf20badcbd3698f33.min.js","sourceRoot":""}

If devtool entry is not present in the webpack config, the map file will properly be generated.

Here is my webpack config. In the console, I execute it like this, npm run build:prod

var path = require('path');
var merge = require('webpack-merge');
var webpack = require('webpack');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var webpackConfig = require('tools-configurations/webpack.config.js');

var isBuildForProd = (process.env.npm_lifecycle_event === 'build:prod');

webpackConfig = merge(webpackConfig, {
  entry: {
    app: [path.resolve(__dirname, 'entry_points', 'app', 'index.js')]
  },
  output: {
    path: path.resolve(__dirname, 'dist', 'app'),
    publicPath: '/app/'
  },
  plugins: [
    new webpack.ProvidePlugin({
      jQuery: 'jquery',
      Promise: 'bluebird',
      Basil: 'basil.js'
    }),
    new HtmlWebpackPlugin({
      title: 'App Entertainment',
      favicon: 'assets/images/favicon.ico',
      inject: 'body',
      template: path.resolve(__dirname, 'node_modules', 'apps-shared', 'assets', 'partials', 'index.html'),
      chunks: ['app']
    })
  ],
  module: {
    loaders: [
      {
        test: /fonts\/.+\.svg(\?v=[0-9]\.[0-9]\.[0-9])?$/,
        loader: 'file-loader?name=assets/fonts/[hash].[ext]'
      }
    ]
  }
});

if (isBuildForProd) {
  webpackConfig = merge(webpackConfig, {
    devtool: 'source-map',
    output: {
      path: path.resolve(__dirname, 'distprod', 'app'),
      publicPath: '/app/'
    },
    plugins: [
      new webpack.SourceMapDevToolPlugin({
        filename: '[file].map',
        append: `\n//# sourceMappingURL=//internal-svr:3333/[url]`
      })
    ]
  });
}

webpackConfig.resolve.root.unshift(
  path.resolve(__dirname, 'node_modules', 'apps-shared', 'overrides')
);

module.exports = webpackConfig;