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

Item property "resource" is not defined for media type "resource".

Item property "resource" is not defined for media type "resource".

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Fix all OPF-012 errors in your EPUB automatically

PreFlight checks for OPF-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 Item property "resource" is not defined for media type "resource".. 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 OPF-012: Item property "resource" is not defined for media type "resource".

Manual Fix Instructions (for Sigil / Calibre)

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