webpack-mergeで設定ファイルを分割しようとしたのですが、Invalid configuration objectというエラーが発生してしまいます

Question

発生している問題

以下の設定でバンドルしようとすると、エラーが発生してしまいます。

const merge = require("webpack-merge");
const common = require("./webpack.common.js");

module.exports = merge(common, {
  mode: "development",
});

// 以下のコードではエラーが発生しませんでした。
// module.exports = merge(common);

× ｢wds｣: Invalid configuration object. Webpack has been initialised using a
configuration object that does not match the API schema.
 - configuration has an unknown property '0'. These properties are valid:
   object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtoo
l?, entry?, externals?, infrastructureLogging?, loader?, mode?, module?, nam
e?, node?, optimization?, output?, parallelism?, performance?, plugins?, pro

file?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolv
eLoader?, serve?, stats?, target?, watch?, watchOptions? }
   For typos: please correct them.
   For loader options: webpack >= v2.0.0 no longer allows custom properties
in configuration.
     Loaders should be updated to allow passing options via loader options i
n module.rules.
     Until loaders are updated one can use the LoaderOptionsPlugin to pass t
hese options to the loader:

     plugins: [
       new webpack.LoaderOptionsPlugin({
         // test: /.xxx$/, // may apply this only for some modules
         options: {
           0: …
         }
       })

     ]