diff --git a/package.json b/package.json
index 4902b99..61e182e 100644
--- a/package.json
+++ b/package.json
@@ -25,7 +25,7 @@
     "vanilla-test": "^1.4.8"
   },
   "scripts": {
-    "prepublish": "esbuild node-ipc.js --bundle --target=es2018 --platform=node --outfile=node-ipc.cjs",
+    "prepublish": "esbuild node-ipc.js --bundle --format=cjs --target=es2018 --platform=node --outfile=node-ipc.cjs",
     "test": "npm i && c8 -r lcov -r html node test/CI.js && c8 report && node ./lcov.js",
     "coverage": "echo 'See your coverage report at http://localhost:8080' && node-http-server port=8080 root=./coverage/"
   },