Error: react-dom/server is not supported in React Server Components; when using @react-email/render in route.ts

I am trying to generate HTML for an email using `render()` from the React Email library within an `app/api/send/route.ts` handler. 

I am seeing this.

```
Error: react-dom/server is not supported in React Server Components.
```

Which is hard to debug since this is a route and not a component.