diff --git a/tasks/build-ext.js b/tasks/build-ext.js
index 18651389c5..7053460a12 100644
--- a/tasks/build-ext.js
+++ b/tasks/build-ext.js
@@ -49,9 +49,7 @@ function main() {
     const options = {
       extend: true,
       entry: require.resolve(ext.module),
-      dest: `${path.join(__dirname, '..', 'build', 'ol.ext', moduleName.toLowerCase())}.js`,
       format: 'iife',
-      moduleName: moduleName,
       exports: 'named',
       plugins: [
         node(),
@@ -60,7 +58,11 @@ function main() {
         wrap(ext)
       ]
     };
-    return rollup(options).then(bundle => bundle.write(options));
+    return rollup(options).then(bundle => {
+      options.name = moduleName;
+      options.file = `${path.join(__dirname, '..', 'build', 'ol.ext', moduleName.toLowerCase())}.js`;
+      bundle.write(options);
+    });
   }));
 }