---
title: "ILHA004: Generator resumed while an instruction was in flight"
description: Ilha tried to drive a generator component again before the instruction it yielded finished.
---

A generator component yields one instruction at a time, such as an Effect or a Stream. Ilha resumes the generator when that instruction finishes. If anything drives the same component again while an instruction is pending, Ilha throws `ILHA004`.

## Why it happens

- Two `mount()` calls target the same running component fiber.
- Custom renderer code calls the runtime's generator driver twice for one fiber.

You rarely see this from ordinary components. It signals that two drivers share one fiber.

## Bad

```tsx
import * as Effect from "effect/Effect";
import { mount } from "ilha";

function* Status() {
  yield Effect.never;
}

// A renderer that re-enters the same fiber while `Effect.never` is pending.
```

## Fixed

Mount each generator component once per host, and let Ilha resume it:

```tsx twoslash
import * as Effect from "effect/Effect";
import { mount } from "ilha";

function* Status() {
  yield <p>loading</p>;
  yield Effect.sleep(100);
  yield <p>ready</p>;
}

mount(document.body, Status);
```