400 Bad Request on POST /api/v2/flows/executions with valid JSON payload

Hitting a 400 error when trying to kick off a flow via the REST API. The endpoint is POST /api/v2/flows/executions. I’ve double-checked the flow ID and the token scopes. Here’s the request body:

{
 "flowId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
 "input": {
 "queueId": "q123",
 "userId": "u456"
 }
}

The response just says validation failed without specifics. Is the input structure wrong?