# OPF-030: unique-identifier not found, how to fix it

Source: https://epubcc.com/epubcheck/opf-030/ (updated 2026-11-04)

EPUBCheck OPF-030 is an error message: “The unique-identifier "…" was not found.” The package element's unique-identifier attribute names an id that no dc:identifier element has. EPUB requires the book's main identifier to be marked this way, so the book has no valid identity until it is fixed.

- Official message (EPUBCheck 5.4.0): The unique-identifier "%1$s" was not found.
- Default severity: Error

## Typical causes

- The id on dc:identifier was renamed or removed while editing the metadata by hand.
- The dc:identifier itself was deleted, leaving the attribute pointing at nothing.
- A typo or a difference in case between the two values.

## How to fix it

1. Read the value of unique-identifier on the package element.
2. Give one dc:identifier the same id, for example <package unique-identifier="uid"> with <dc:identifier id="uid">urn:uuid:…</dc:identifier>.
3. If the book has no identifier at all, add one (a UUID or an ISBN).

## When it's not this

If the attribute is missing from the package element, that is OPF-048.

## Sources

- EPUB 3.3, §5.4 The package element: https://www.w3.org/TR/epub-33/#sec-package-elem (read 2026-11-04)
- EPUB 3.3, §5.5.3.1.1 The dc:identifier element: https://www.w3.org/TR/epub-33/#sec-opf-dcidentifier (read 2026-11-04)

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EpubCC.