EPUBCheck
EPUBCheck evaluates EPUB 2 and EPUB 3 publications against official EPUB specifications and reports structure, package, markup, and reference problems.
EPUB error guide
RSC-012 EPUB fragment error
What RSC_012 fragment identifier errors mean and when broken EPUB anchors can be repaired safely.
Error text
RSC_012: Fragment identifier is not defined.
When you usually see it
KDP or EPUBCheck finds a link such as chapter.xhtml#note-4, but the target XHTML file has no matching id.
Plain English
What it means inside the EPUB.
The file exists, but the exact anchor inside that file does not.
This often happens after headings, footnotes, endnotes, or TOC entries are edited and the old id remains in a link.
User checklist
What to inspect before another upload.
These checks keep the repair decision tied to the EPUB package instead of guessing from a generic error code.
- Split the broken link into the XHTML file path and the fragment after the # symbol.
- Open the target XHTML file and check whether a matching id value exists.
- Treat footnotes, endnotes, and repeated heading text as manual review unless one target is clearly correct.
Standards context
Why this error matters to validators.
EPUB 3.3 package document
The package document defines the metadata, manifest, spine, and navigation wiring that validators inspect before a reading system can process the book reliably.
Repairability
Can EPUB Fixer repair it?
Safe repair condition
EPUB Fixer can repair a fragment only when there is one clear target, such as a moved body id or an obvious heading id mismatch.
When it stops at diagnosis
Footnotes, endnotes, and duplicated heading text often require manual review because choosing the wrong anchor can send readers to the wrong place.
Before / after example
Before: nav.xhtml links to chapter2.xhtml#section-5, but chapter2.xhtml contains id="sec-5". After: the link is updated only if that is the single safe match.
Next step
Scan the actual file before editing by hand.
The same error code can come from different files and different causes. Uploading the EPUB lets the report show the affected path and whether the fix is deterministic.
Upload EPUB to scanFAQ
Common questions about this EPUB error.
What does RSC-012 fragment identifier is not defined mean?
It means the EPUB link points to a file that exists, but the anchor after the # symbol does not exist inside that file. This often affects table of contents links, footnotes, endnotes, or heading links.
How do I fix an EPUB anchor that KDP cannot find?
Compare the link target with the id values inside the target XHTML file. A safe fix requires one clear matching anchor; otherwise the link may send readers to the wrong section.
Why does RSC-012 happen after editing a book?
Editing tools can rename headings, footnotes, or section ids while old links remain in the nav, NCX, or chapter files. The file path still works, but the specific fragment target is gone.
Can EPUB Fixer repair RSC-012 automatically?
Only when there is a single obvious replacement anchor, such as a moved body id or a simple heading id mismatch. Footnotes and duplicated headings usually need manual review.