diff --git a/packages/webchannel-wrapper/gulpfile.js b/packages/webchannel-wrapper/gulpfile.js
index de51bc0ded1..8148bb036bb 100644
--- a/packages/webchannel-wrapper/gulpfile.js
+++ b/packages/webchannel-wrapper/gulpfile.js
@@ -120,7 +120,7 @@ function createRollupTask({
     };
 
     const outputOptions = {
-      file: `dist/index${outputExtension ? '.' : ''}${outputExtension}.js`,
+      file: `dist/index${outputExtension ? '.' : ''}${outputExtension}.mjs`,
       format,
       sourcemap: true,
       // Prevents warning when compiling CJS that there are named and default exports together.
diff --git a/packages/webchannel-wrapper/package.json b/packages/webchannel-wrapper/package.json
index 35217bc539f..3c23acef835 100644
--- a/packages/webchannel-wrapper/package.json
+++ b/packages/webchannel-wrapper/package.json
@@ -10,8 +10,8 @@
     ".": {
       "types": "./src/index.d.ts",
       "require": "./dist/index.js",
-      "esm5": "./dist/index.esm.js",
-      "default": "./dist/index.esm2017.js"
+      "esm5": "./dist/index.esm.mjs",
+      "default": "./dist/index.esm2017.mjs"
     },
     "./package.json": "./package.json"
   },