diff --git a/tsup.config.ts b/tsup.config.ts
index dcf7c4e..9e37a8c 100644
--- a/tsup.config.ts
+++ b/tsup.config.ts
@@ -1,5 +1,5 @@
-import { defineConfig, Options } from 'tsup'
-import fs from 'fs'
+import type { Options } from 'tsup'
+import { defineConfig } from 'tsup'
 
 export default defineConfig(options => {
   const commonOptions: Partial<Options> = {
@@ -16,13 +16,15 @@ export default defineConfig(options => {
       outExtension: () => ({ js: '.mjs' }),
       dts: true,
       clean: true,
-      onSuccess() {
-        // Support Webpack 4 by pointing `"module"` to a file with a `.js` extension
-        fs.copyFileSync(
-          'dist/redux-thunk.mjs',
-          'dist/redux-thunk.legacy-esm.js',
-        )
-      },
+    },
+    // Support Webpack 4 by pointing `"module"` to a file with a `.js` extension
+    {
+      ...commonOptions,
+      format: ['esm'],
+      target: 'es2017',
+      dts: false,
+      outExtension: () => ({ js: '.js' }),
+      entry: { 'redux-thunk.legacy-esm': 'src/index.ts' },
     },
     {
       ...commonOptions,