# EPUBCheck OPF-028: Undeclared prefix: …

Source: https://epublab.com/epubcheck/opf-028/ (updated 2026-11-03)

EPUBCheck OPF-028 is an error message: “Undeclared prefix: "…".” A property uses a prefix that is not declared in the package document's prefix attribute.

- Official message (EPUBCheck 5.4.0): Undeclared prefix: "%1$s".
- Default severity: Error

## Typical causes

- Vendor properties such as ibooks:version used without declaring the ibooks prefix.

## How to fix it

- The prefix attribute on the package element maps short prefixes to vocabulary URIs, as space-separated pairs like prefix="ibooks: http://vocabulary.itunes.apple.com/rdf/ibooks/vocabulary-extensions-1.0/". Add the mapping for the prefix, or remove the property.

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