400 Bad Request on POST /api/v2/flows/executions with external trigger

Getting a 400 error when trying to kick off an Architect flow from our Node.js backend. The endpoint is /api/v2/flows/executions. Here’s the payload:

{"flowId": "abc-123", "triggerType": "Web", "attributes": {"custom": "data"}}

The response says triggerType is invalid. I’ve checked the docs and “Web” should work. Is there a specific format for the triggerType when calling this from an external API? The flow is published and active.