Back to Error DirectoryW3C EPUBCheck Standard
CSS-002Retailer Warning

Empty or NULL reference found.

Empty or NULL reference found.

Automated 1-Click Fix Ready

Fix all CSS-002 errors in your EPUB automatically

PreFlight checks for CSS-002 issues in memory and provides 1-click remediation during client-side packaging.

Scan & Fix Now

What This Error Means in Plain English

EPUBCheck detected Empty or NULL reference found.. 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 CSS-002: Empty or NULL reference found.

Manual Fix Instructions (for Sigil / Calibre)

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