Outbound contact list POST 400 INVALID_VALUE on name field

Hitting a 400 error when trying to create a contact list via the outbound API. The docs for POST /api/v2/outbound/contactlists say the name field is required and should be a string. I’m sending a valid string.

POST /api/v2/outbound/contactlists
Authorization: Bearer <token>
Content-Type: application/json

{
 "name": "Test List Alpha",
 "description": "Auto generated"
}

Response is just:

{
 "code": "invalid_value",
 "message": "The request body was not valid.",
 "status": 400
}

No nested error details. Checked the schema validator, JSON is valid. Is there a hidden constraint on the name field length or characters that the API docs missed? Or is the endpoint rejecting the request before it even parses the body properly?