POST /api/v2/outbound/contactlists returns 400 INVALID_VALUE on valid JSON

Trying to push a new contact list via the REST API. The docs state: “The contact list name must be unique within the organization.” I’m sending a unique name, valid CSV format, and the correct region header.

Still getting a 400 Bad Request. The error payload is generic.

{
 "errors": [
 {
 "code": "INVALID_VALUE",
 "message": "Invalid value"
 }
 ]
}

Here is the payload I’m POSTing to /api/v2/outbound/contactlists.

{
 "name": "Campaign_Test_List_001",
 "description": "Initial load for testing",
 "type": "CALLABLE",
 "contacts": [
 {
 "id": "1",
 "contactKey": "contact_key_1",
 "fields": {
 "firstName": "John",
 "lastName": "Doe",
 "email": "john.doe@example.com"
 }
 }
 ]
}

Headers include Content-Type: application/json and Accept: application/json. Authorization is a fresh Bearer token from client credentials.

The field mapping in the org matches the keys. No special characters in the name. Why is it rejecting this as invalid?