---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/16024#article
headline: "16024: Participant label invalid: max 128 chars, not a CallSid, no '/'"
url: https://www.twilio.com/docs/api/errors/16024
inLanguage: en
dateModified: 2026-04-10T12:20:38.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 16024: Participant label invalid: max 128 chars, not a CallSid, no '/'

Log Type: APPLICATION

Log Level: ERROR

## Description

You receive this error when you send a `POST` request to create a Conference participant with an invalid `Label`. The value must be 128 characters or fewer, it cannot resemble a Call SID, and it cannot contain `/`, including `%2F`.

### Possible causes

* Your request sets `Label` to a value longer than 128 characters.
* Your request uses a Call SID, or a value that resembles a Call SID, as the participant label.
* Your request includes `/` in the `Label` value.
* Your application URL-encodes `/` as `%2F` and sends it as part of the label. This is still invalid.

### Possible solutions

* Validate `Label` before you create the participant. Keep the value at 128 characters or fewer.
* Use a custom participant label instead of a Call SID. You can use that label later to fetch, update, or delete the participant.
* Remove `/` from your label generation logic. Do not rely on `%2F` as a workaround because Twilio rejects that value too.
* If you use labels in participant URLs later, percent-encode other non-URL-safe characters in the request path. For example, encode spaces as `%20`.

#### Additional resources

* [Conferences Participants subresource](/docs/voice/api/conference-participant-resource)
* [Voice Conference](/docs/voice/conference)
* [Create Conference Calls](/docs/voice/tutorials/how-to-create-conference-calls)