400 INVALID_VALUE on POST /api/v2/outbound/contactlists

Looking for advice on INVALID_VALUE when POSTing to /api/v2/outbound/contactlists. I’m building the payload with Python requests. The contact list name is unique, status is ACTIVE. Redis cache is cleared. Still getting 400.

payload = {
 "name": "test_list_01",
 "status": "ACTIVE",
 "contactListType": "IMPORT"
}

Is there a hidden required field for IMPORT type?