---
title: MARKLESS_CALLBACK_SLOT_UNBOUND
description: "A callback slot is invoked but no component fills it: a Markless compiler warning."
sidebar: { hidden: true }
search: { keywords: ["CALLBACK_SLOT_UNBOUND","callback slot unbound","markless error"] }
pagination: false
---

**Warning** from the compiler, while it builds your `.tsrx` files.

## What happened

Markless prints the message, the reason, and a suggestion:

```text
MARKLESS_CALLBACK_SLOT_UNBOUND: shared() "<definitionName>" invokes its "<slotName>" callback slot, but no component in this module assigns a callback prop into it.
Without an assignment there is no widget root to route the call to, so the invocation can never reach a consumer and does nothing at runtime.
Assign the widget root's callback prop into the slot: <instance>.<slotName> = <thatProp>.
https://markless.dev/errors/MARKLESS_CALLBACK_SLOT_UNBOUND
```

Words in angle brackets change from case to case.

## How to fix it

Start with the suggestion in the message above.

If you accept the warning on purpose, put this comment on the line above it:

```tsrx
// markless-allow MARKLESS_CALLBACK_SLOT_UNBOUND: the reason you accept it
```

This page has no guide yet. [Help write it](/contributing/improve-these-docs).

## Where it comes from

`@markless/compiler` raises this code. Source:

- [packages/compiler/src/passes/semantic-graph/diagnostics.ts](https://github.com/compiled-run/markless/blob/main/packages/compiler/src/passes/semantic-graph/diagnostics.ts)

Read [how to read a Markless error](/tooling/diagnostics), or go back to [all error codes](/errors).