`event.$fetch` returns HTTP 500, whereas `$fetch` works fine

[DamianGlowala]

Environment

n/a

Reproduction

https://stackblitz.com/edit/github-uhgm3j-qvnqae?file=server%2Fapi%2Ftest.post.ts

Describe the bug

Observe how calling the endpoint in server route with $fetch works correctly, and how it breaks with event.$fetch by returning HTTP 500.

Additional context

No response

Logs

No response