While developing an app I've used a custom server setup with express and I also used fetch.
I had no idea that the latest version of Safari doesn't yet support fetch natively and it took me a while to understand this, so maybe it's possible to add a more friendlier error message?
I ended up using isomorphic-fetch 💯

Error message running in dev mode (const app = next({ dir: '.', dev: true });) :

Error message running in prod mode (const app = next({ dir: '.', dev: false });) :