POST /api/v2/outbound/contactlists returns 400 INVALID_VALUE on name field

Trying to create a new outbound contact list via the API. Getting a 400 Bad Request with INVALID_VALUE. The error points to the name field, but it’s just a simple string.

{
 "name": "Test List 1",
 "label": "Test",
 "description": "API test"
}

I’ve checked the docs and the name is valid. Tried changing it to alphanumeric only. Same error. Is there a hidden uniqueness constraint or format rule I’m missing?