# EPUBCheck OPF-048: missing unique-identifier, how to fix it

Source: https://epubcc.com/epubcheck/opf-048/ (updated 2026-11-04)

EPUBCheck OPF-048 is an error message: “Package tag is missing its required unique-identifier attribute and value.” The package element has no unique-identifier attribute, which is required. It points to the dc:identifier that identifies the book, and reading apps use it to tell books and editions apart.

- Official message (EPUBCheck 5.4.0): Package tag is missing its required unique-identifier attribute and value.
- Default severity: Error

## Typical causes

- A package document written by hand or by a tool that omitted it.
- The attribute misspelled, such as uniqueidentifier or unique-id.
- A package element replaced when merging books.

## How to fix it

1. Make sure the metadata has a dc:identifier with an id, for example <dc:identifier id="uid">urn:uuid:…</dc:identifier>.
2. Add unique-identifier with that id to the package element, for example <package version="3.0" unique-identifier="uid">.
3. Run EPUBCheck again to confirm the two match.

## When it's not this

If the attribute exists but points to an id no dc:identifier has, that is OPF-030.

## Sources

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

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