400 Error when setting wrap-up code via API on ended interaction

Getting a 400 Bad Request when trying to push a wrap-up code to an already ended interaction. The PUT /api/v2/conversations/interactions/{id} endpoint rejects the payload. Here’s what I’m sending:

{
 "wrapup_code": {
 "id": "12345", 
 "label": "Resolved"
 }
}

The docs say this should work for post-interaction cleanup, but I’m getting Interaction has already ended. How do I actually set this programmatically without breaking the trace span?