POST /api/v2/flows/executions returns 400 Bad Request

NICE CXone API & Integrations
1

Trying to launch an Architect flow from our WFM adherence script. Getting a 400 Bad Request on POST /api/v2/flows/executions. Here is the payload:

{
 "flowId": "abc-123",
 "externalContactId": "ext-456",
 "parameters": {
 "agentId": "789"
 }
}

The error message is just "message":"Invalid input". Do I need to set a specific executionContext or am I missing a required parameter?