# EPUBCheck RSC-012: Fragment identifier is not defined

Source: https://epublab.com/epubcheck/rsc-012/ (updated 2026-11-03)

EPUBCheck RSC-012 is an error message: “Fragment identifier is not defined.” A link points to a fragment (#id) that does not exist in the target document.

- Official message (EPUBCheck 5.4.0): Fragment identifier is not defined.
- Default severity: Error

## Typical causes

- Footnote or table of contents links to an id that was renamed or never written, a common result of conversion and of splitting a book into chapters.
- A link that targets the right id but the wrong file after a chapter was split.

## How to fix it

- Add the missing id to the target element, or correct the link to an id that exists.
- After splitting chapters, update links so they name the file where the id now lives.

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EPUBLab.