# 40006: Message contains invalid connection ID

Identifier: `message_contains_invalid_connection_id`



A published message referenced a connection by an ID or key that was invalid or did not match, so the message was rejected.

## What you should do

When [publishing on behalf of a realtime connection](https://ably.com/docs/pub-sub/advanced.md#publish-on-behalf), make sure the `connectionKey` you publish with is the current key of that connection, and that any `connectionId` set on a message matches the publishing connection. Correct the mismatched value and resend.

## Why it happens

A published message referenced a connection that couldn't be validated. Usual causes are a malformed or stale `connectionKey`, a REST publisher and realtime client that belong to different Ably apps or use different client IDs, or a manually constructed message whose `connectionId` doesn't match the current connection.

## What you'll see

The error is reported with code 40006 and HTTP status 400. The message is typically `Malformed message; invalid connectionId` or `Malformed message; mismatched connectionId`.