Back to Error DirectoryW3C EPUBCheck Standard
PKG-017Critical Ingestion Error

Uncommon EPUB file extension.

Uncommon EPUB file extension.

Automated 1-Click Fix Ready

Fix all PKG-017 errors in your EPUB automatically

PreFlight checks for PKG-017 issues in memory and provides 1-click remediation during client-side packaging.

Scan & Fix Now

What This Error Means in Plain English

EPUBCheck detected Uncommon EPUB file extension.. 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-017: Uncommon EPUB file extension.

Manual Fix Instructions (for Sigil / Calibre)

  1. Locate the file and line flagged under rule PKG-017 in your EPUB editor (Sigil or Calibre).
  2. Apply the required syntax correction matching the PKG-017 specification.
  3. Re-pack and validate with PreFlight EPUB to confirm zero residual errors.