Epub error codes, explained

Guides to the validation errors people actually hit: what each message means, why it happens, and how to fix it. Every guide pairs with the checker, which finds these problems in your browser and repairs the mechanical ones in one click.

By error code

• PKG-006: mimetype file entry is missing or not first in the archive. The most common packaging error, and auto-fixable.
• RSC-005: the catch-all for content rule violations. The message text carries the real problem; this guide decodes the common ones.
• RSC-007: a referenced resource is not in the book. Blank images and unstyled chapters.
• RSC-016: an element is never closed, so parsing stops dead. Fatal, and the cause of most error avalanches.
• NCX-001: the NCX identifier does not match the package identifier. A Calibre-edit classic, and auto-fixable.
• OPF-029: a file does not match the media type the manifest declares, usually an image in disguise.
• PKG-022: an image has the wrong file extension. OPF-029's warning-level sibling.

By situation

• Validation errors in a Kindle-converted Epub: page-map, data-AmznRemoved, incomplete pageList blocks, and the other Amazon leftovers.
• KDP rejected your Epub: how to find the actual problem when Amazon's error message will not tell you.
• Apple Books rejected your Epub: Apple requires a clean Epubcheck run; here is how to get one before you deliver.

Your code not here?

Run the book through the checker anyway. It explains every problem it finds in plain language, whatever the code, and shows the exact source line. And if you hit an error you think deserves a guide, tell us: hello@ebooknexus.com.