diff --git a/src/compiler/config/test/validate-config.spec.ts b/src/compiler/config/test/validate-config.spec.ts
index 1a1060988dd..c99446e3cc2 100644
--- a/src/compiler/config/test/validate-config.spec.ts
+++ b/src/compiler/config/test/validate-config.spec.ts
@@ -86,14 +86,15 @@ describe('validation', () => {
       expect(config.buildEs5).toBe(true);
     });
 
-    it('prod mode default to both es2017 and es5', () => {
+    it('prod mode, set modern and es5', () => {
       userConfig.devMode = false;
+      userConfig.buildEs5 = true;
       const { config } = validateConfig(userConfig);
       expect(config.buildEs5).toBe(true);
     });
 
-    it('dev mode default to only es2017', () => {
-      userConfig.devMode = true;
+    it('prod mode default to only modern and not es5', () => {
+      userConfig.devMode = false;
       const { config } = validateConfig(userConfig);
       expect(config.buildEs5).toBe(false);
     });
diff --git a/src/compiler/config/validate-config.ts b/src/compiler/config/validate-config.ts
index bfebb2cb9a0..d03aa89ec65 100644
--- a/src/compiler/config/validate-config.ts
+++ b/src/compiler/config/validate-config.ts
@@ -41,13 +41,14 @@ export const validateConfig = (userConfig?: Config) => {
   config.extras.initializeNextTick = !!config.extras.initializeNextTick;
   config.extras.tagNameTransform = !!config.extras.tagNameTransform;
 
+  config.buildEs5 = !!config.buildEs5;
+
   setBooleanConfig(config, 'minifyCss', null, !config.devMode);
   setBooleanConfig(config, 'minifyJs', null, !config.devMode);
   setBooleanConfig(config, 'sourceMap', null, false);
   setBooleanConfig(config, 'watch', 'watch', false);
   setBooleanConfig(config, 'minifyCss', null, !config.devMode);
   setBooleanConfig(config, 'minifyJs', null, !config.devMode);
-  setBooleanConfig(config, 'buildEs5', 'es5', !config.devMode);
   setBooleanConfig(config, 'buildDocs', 'docs', !config.devMode);
   setBooleanConfig(config, 'buildDist', 'esm', !config.devMode || config.buildEs5);
   setBooleanConfig(config, 'profile', 'profile', config.devMode);