# EPUBCheck RSC-012: broken fragment link (#id), how to fix it

Source: https://epubcc.com/epubcheck/rsc-012/ (updated 2026-11-04)

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. The link opens the right file but cannot jump to the intended place, which is common with footnotes and table of contents entries.

- 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

1. Find the link and the id named in the message.
2. Add the missing id to the target element, or correct the link to an id that exists.
3. After splitting chapters, update links so they name the file where the id now lives.

## When it's not this

If the file itself is missing, that is RSC-007; a fragment on a PNG or JPEG is RSC-009.

## Sources

- EPUB 3.3, §4.2.5 URLs in the OCF abstract container: https://www.w3.org/TR/epub-33/#sec-container-iri (read 2026-11-04)

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