TypeError: Cannot read property 'push' of undefined

preact.config.js:

```
import asyncPlugin from 'preact-cli-plugin-async';

export default (config, env, helpers) => {
asyncPlugin(config);
};

```

error:

```
npm run build
> test-widget@0.1.0 build /media/sdb1/dev/preact/test-widget
> preact build --no-prerender --clean --template src/index.ejs --service-worker false

Error: Error at /media/sdb1/dev/preact/test-widget/preact.config.js:
TypeError: Cannot read property 'push' of undefined

```