Back to Error DirectoryW3C EPUBCheck Standard
CHK-002Retailer Warning

Unrecognized custom message id resource encountered in message over...

Unrecognized custom message id resource encountered in message overrides file "resource".

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PreFlight checks for CHK-002 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 Unrecognized custom message id resource encountered in message overrides file "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 CHK-002: Unrecognized custom message id resource encountered in message overrides file "resource".

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule CHK-002 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the CHK-002 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.