fix: Fastify no longer complains about the type

[EDM115]

aims to fix #39

Tested with Fastify 5.x, works well.
Test passes.
I have no idea however if it works for older versions of Fastify and other frameworks.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[adrai]

thank you for your contribution, included in v3.7.2