EPUB error guide
Use this when an EPUB link reaches the right XHTML file but the anchor after # is missing. Check the file, the missing id, and where the reader should land before changing the link.
Read the link
The part before # names the XHTML file. The part after # names the missing anchor inside that file.
Check the target
Automatic repair is limited to one existing id that differs only by capitalization or URL encoding. Similar wording, hyphens, or nearby text are not enough.
Stop if unclear
Do not add an id only to silence the error. The link must still take the reader to the intended note, heading, or return point.
Scan when the message does not name the affected path. The report shows affected paths and repair decisions. Your original EPUB stays unchanged.
Need fixed-layout, DRM, or source-file help? Check unsupported or review-first EPUB cases.
Match these report titles
Start here
In an EPUB report, RSC_012 is easy to misread as a missing chapter. Usually the XHTML file exists; the problem is the anchor after the #.
The target XHTML file exists, but the exact anchor inside that file does not. This is different from a missing file error: the path before the # resolves, while the fragment after the # points to an id that is absent, renamed, duplicated, or located in a different file.
EPUB Fixer can repair a fragment only when exactly one existing id in the target file differs from the requested fragment by capitalization, URL encoding, or both. The repaired EPUB must then pass EPUBCheck again.
Split the broken link into two parts: the file path before # and the anchor name after #.
Footnotes, endnotes, repeated headings, and duplicated section titles often require manual review because choosing the wrong anchor can send readers to the wrong note, chapter section, or backlink. Do not create a new anchor if you cannot prove where the reader should land.
1. Example report output
The useful detail is not just the code. It is the file before the # symbol, the missing fragment after it, and whether the link is navigation, a footnote, or an ordinary section jump.
EPUBCheck message
ERROR(RSC-012): OEBPS/nav.xhtml(18,32): Fragment identifier is not defined.
KDP / Previewer wording
A table of contents, footnote, or internal link points to a missing location.
EPUB Fixer report
Source link OEBPS/nav.xhtml, target file OEBPS/Text/chapter2.xhtml, missing fragment #note-4.
Do this next
Open the target XHTML file and confirm the intended id before adding or renaming an anchor.
Do not do this
Do not add #note-4 to the top of the chapter unless that is where the reader should land.
Automatic repair is safe only when exactly one existing id matches by capitalization or URL encoding. Renamed headings, footnotes, and backlinks need manual context.
2. Next step
RSC_012 is usually an anchor problem. Split the target into file path and fragment before deciding whether to scan, inspect manually, or fix it in the source tool or EPUB editor.
Upload scan
RSC_012 means the file before # exists, but the id after # is missing inside that target file. The report should show the source link, the target XHTML file, and whether the missing fragment belongs to a TOC entry, chapter link, footnote, endnote, or backlink.
Manual check
For footnotes, backlinks, or repeated headings, the correct fix depends on where the reader should land. Open the target XHTML file, compare the nearby text around the missing id, and only add or rename an id when the exact reading location is clear.
Repair summary
The report can repair one capitalization or URL-encoding difference. If the id was renamed, shortened, moved, or replaced semantically, use the source file, target file, missing fragment, and nearby text for manual correction.
3. Concrete path example
The file can exist while the anchor after the # is still missing.
Message: ERROR(RSC-012): OEBPS/nav.xhtml points to Text/chapter2.xhtml#SECTION-5 Report shows: file exists, missing id SECTION-5 Existing id: section-5 Fix: change only the fragment capitalization, then validate again
This can be automatic only because exactly one existing id differs by case. A different spelling, shortened id, or nearby heading still requires manual review.
Quick decision
The file before the # and the fragment after it must both be checked. The same missing id can be harmless in a section link and risky in a footnote loop.
Scan first
Scan the EPUB to locate the source link and target XHTML file, then decide whether the missing fragment is a heading, TOC target, footnote, or backlink.
Safe fix
Update the fragment only when exactly one existing id is the same value after case-insensitive or URL-encoding comparison. Do not infer replacements from similar names or nearby content.
Manual review
Footnotes and repeated section titles are easy to mis-target. Do not create a new id unless you know exactly where the reader should land.
Common situations
RSC_012 always points after a # symbol, but the source of that link determines the risk.
The chapter file exists, but the heading id after # changed during export or editing, so the TOC opens a location that no longer exists.
Repair automatically only when exactly one existing id differs by case or URL encoding. Any real rename needs manual confirmation.
The note may have been renumbered, moved to an endnotes file, or split into another chapter, so adding the old id in the wrong place can send readers to the wrong note.
Check both the forward note link and the return backlink before changing ids.
The note itself can exist while the return target no longer exists in the chapter.
Do not point every backlink to the chapter top; choose the original reading location or review manually.
Export tools can generate similar ids for repeated headings such as Introduction, Notes, or Chapter Summary.
Stop for manual review unless one target is clearly tied to the source link.
Some exports leave links such as chapter.xhtml#bodymatter after the chapter body id was removed or renamed.
Check the target file for the current body or heading id. Do not replace it with the first id you find.
RSC_012: Fragment identifier is not defined.
KDP, Kindle Previewer, or EPUBCheck finds a link such as chapter.xhtml#note-4, chapter2.xhtml#section-5, or nav.xhtml#toc, but the target XHTML file has no matching id attribute for the fragment after the # symbol.
What it means
This often happens after headings, footnotes, endnotes, or TOC entries are edited and the old id remains in a nav file, NCX file, chapter link, or footnote backlink. Export tools can also regenerate heading ids while preserving older links from a previous EPUB build.
Before you edit
A safe repair keeps the reader in the same place the original link intended. That is why footnotes, endnotes, repeated headings, and backlinks need more caution than ordinary section anchors.
Why KDP checks it
EPUBCheck checks EPUB 2 and EPUB 3 files against the official rules and reports package, markup, link, and file-reference problems.
The OPF file is the EPUB's map: metadata, file list, reading order, and navigation wiring. KDP and EPUBCheck use it to decide whether the book package is coherent.
Repair example
Before: nav.xhtml links to chapter2.xhtml#SECTION-5, but chapter2.xhtml contains one id="section-5". After: correct only that capitalization difference and validate again. Before: a footnote backlink points to #fnref12 but the chapter has several note references; that stays manual.
Ready to retry?
The scan shows whether the broken fragment is in a TOC entry, a footnote backlink, an endnote, or an ordinary chapter link, which changes the safe next step.
Upload EPUB to scanFAQ
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.
Compare the fragment with id values inside the target XHTML file. Automatic repair requires exactly one existing id that differs only by capitalization or URL encoding. Any renamed, moved, footnote, backlink, or semantic replacement needs manual confirmation.
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.
Only when exactly one existing id differs from the missing fragment by capitalization, URL encoding, or both. Renamed or moved anchors, footnotes, backlinks, and duplicated headings need manual review.
A TOC or nav file can point to a chapter file and a specific heading id. If the chapter file still exists but the heading id changed during export, the TOC entry becomes a broken fragment even though the chapter itself is present.
Only when you know the exact reading location the link should target. Adding a new id just to satisfy the validator can create a valid but misleading link, especially for footnotes, endnotes, and repeated headings.
Footnotes often have forward links and backlinks. If the wrong id is chosen, the EPUB may validate while readers jump to the wrong note or cannot return to the text.
