diff --git a/package/__tests__/development.js b/package/__tests__/development.js
index 827667d72..3997602c1 100644
--- a/package/__tests__/development.js
+++ b/package/__tests__/development.js
@@ -14,7 +14,7 @@ describe('Development environment', () => {
     test('should use development config and environment including devServer if WEBPACK_DEV_SERVER', () => {
       process.env.RAILS_ENV = 'development'
       process.env.NODE_ENV = 'development'
-      process.env.WEBPACK_DEV_SERVER = 'YES'
+      process.env.WEBPACK_DEV_SERVER = 'true'
       const { webpackConfig } = require('../index')
 
       expect(webpackConfig.output.path).toEqual(resolve('public', 'packs'))
diff --git a/test/webpacker_test.rb b/test/webpacker_test.rb
index 2aea64014..0fd2d4d35 100644
--- a/test/webpacker_test.rb
+++ b/test/webpacker_test.rb
@@ -23,8 +23,8 @@ def dev_server.pretty?; false; end
     def dev_server.https?; true; end
     def dev_server.hmr?; true; end
     def dev_server.running?; true; end
-    Webpacker::DevServer.stub(:new, dev_server) do
-      assert !Webpacker.inlining_css?
+    Webpacker.instance.stub(:dev_server, dev_server) do
+      assert Webpacker.inlining_css?
     end
   end
 end