diff --git a/.vscode/settings.json b/.vscode/settings.json
index a326c24..b811ff0 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -23,5 +23,5 @@
   "editor.formatOnPaste": false,
   "editor.formatOnType": false,
   "editor.formatOnSave": true,
-  "workbench.colorTheme": "FireFly Pro Bright"
+  "workbench.colorTheme": "Sublime Material Theme - Light"
 }
diff --git a/dist/index.js b/dist/index.js
index a3ceb63..53b5397 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -338,6 +338,8 @@ const npmInstallAction = async () => {
     // From: https://github.com/actions/cache/blob/a2ed59d39b352305bdd2f628719a53b2cc4f9613/src/saveImpl.ts#L96
     if (process.env.NODE_ENV !== 'test') {
       process.exit(0)
+    } else {
+      core.debug('skip process.exit(0) in test mode')
     }
   } catch (err) {
     console.error(err)
diff --git a/index.js b/index.js
index 259155c..e0f1f0f 100644
--- a/index.js
+++ b/index.js
@@ -331,6 +331,8 @@ const npmInstallAction = async () => {
     // From: https://github.com/actions/cache/blob/a2ed59d39b352305bdd2f628719a53b2cc4f9613/src/saveImpl.ts#L96
     if (process.env.NODE_ENV !== 'test') {
       process.exit(0)
+    } else {
+      core.debug('skip process.exit(0) in test mode')
     }
   } catch (err) {
     console.error(err)
diff --git a/package.json b/package.json
index 96d1b95..477c447 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,7 @@
   "scripts": {
     "format": "prettier --write './**/*'",
     "format:check": "prettier --check './**/*'",
-    "test": "NODE_ENV=test npm run unit",
+    "test": "npm run unit",
     "unit": "mocha test/helper 'test/*spec.js'",
     "pretest": "npm run build",
     "build": "ncc build -o dist index.js",
diff --git a/test/helper.js b/test/helper.js
index 5976797..d68bd9b 100644
--- a/test/helper.js
+++ b/test/helper.js
@@ -1,3 +1,6 @@
+// https://github.com/bahmutov/npm-install/pull/228
+process.env.NODE_ENV = 'test'
+
 // following guide
 // https://glebbahmutov.com/blog/mocha-and-sinon/
 const chai = require('chai')