fetch return undefined on iPhone

I have a really basic code

const response = await fetch('/api/user');
const user = await response.json();

And from Sentry I see the following error undefined is not an object (evaluating 'response.json')

I am unable to reproduce it, seems like it happens only on iPhone on any browser. I tried on iPhone and I have no issue.

Furthermore, I can just add a condition to check typeof response === 'undefined' but I would like to understand, from the fetch documentation it should never return undefined