Back to Error DirectoryW3C EPUBCheck Standard
RSC-012Critical Ingestion Error

Fragment identifier is not defined.

Fragment identifier is not defined.

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Fix all RSC-012 errors in your EPUB automatically

PreFlight checks for RSC-012 issues in memory and provides 1-click remediation during client-side packaging.

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What This Error Means in Plain English

EPUBCheck detected Fragment identifier is not defined.. Digital reading systems and bookstores require standard conformance to ensure compatibility across all reading apps.

Why Retailers & EAA Regulations Block This File

Automated book distributors (Apple Books, Amazon KDP, Kobo, and IngramSpark) run strict EPUBCheck validation on ingest and reject non-compliant files.

Technical Specification: W3C EPUBCheck rule RSC-012: Fragment identifier is not defined.

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule RSC-012 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the RSC-012 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.