Back to Error DirectoryW3C EPUBCheck Standard
PKG-016Critical Ingestion Error

Use only lowercase characters for the EPUB file extension for maxim...

Use only lowercase characters for the EPUB file extension for maximum compatibility.

Automated 1-Click Fix Ready

Fix all PKG-016 errors in your EPUB automatically

PreFlight checks for PKG-016 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 Use only lowercase characters for the EPUB file extension for maximum compatibility.. 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 PKG-016: Use only lowercase characters for the EPUB file extension for maximum compatibility.

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule PKG-016 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the PKG-016 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.