# missing_header

> A required header is missing.

Source: https://tickwire.io/docs/errors/missing_header/

HTTP 400.

**Likely causes:**

- X-Actor, X-Reason or Idempotency-Key is missing on a mutating call.
- X-Actor, X-Reason or Idempotency-Key is missing when changing the API version pin.

**What to do:** Send the headers named in the message.

**Retry:** No. The same request gets the same answer; change something first.

Listed under [Requests and validation](https://tickwire.io/docs/errors/#requests-and-validation) in [Errors](https://tickwire.io/docs/errors/). Send the error's `requestId` to support and we can find the request.