'Cannot find preset schema for namespace' when using the API

[Mcdostone]

Hi,

I'm trying validate my RSS file using curl but for some reasons, it doesn't work (I am able to validate HTML, CSS though).
I didn't find any piece of documentation regarding the the w3c validation API.
When I validate the RSS manually using https://validator.w3.org/feed/check.cgi, it works like a charm.

Is there something wrong with my curl command?

Error message
Cannot find preset schema for namespace: “http://www.w3.org/2005/Atom”

Command

curl --request POST \
  --url 'https://validator.w3.org/nu/?out=json' \
  --header 'Content-Type: application/xml' \
  --data '<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
</feed>'

# output
{"messages":[{"type":"non-document-error","subType":"schema","message":"Cannot find preset schema for namespace: “http://www.w3.org/2005/Atom”."}]}