```python
from fastapi import FastAPI, HTTPException
import helpers

app = FastAPI()


@app.post("/reports")
def create_report(payload: dict):
    rows = payload.get("rows")
    if rows is None:
        raise HTTPException(status_code=400, detail="'rows' field is required")
    rendered_report = helpers.render_report(rows)
    return {"length": len(rendered_report)}
```