Outbound contact list POST returning INVALID_VALUE with valid payload

Trying to create a new outbound contact list via the Genesys Cloud API. Getting a 400 INVALID_VALUE error on POST to /api/v2/outbound/contactlists.

POST /api/v2/outbound/contactlists
{
 "name": "test_list",
 "description": "trace test",
 "addressBooks": [{"id": "123"}]
}

The address book ID exists. Trace context is attached via headers but that shouldn’t block creation. Any idea what field is throwing this off?