# EPUBCheck OPF-028: prefix used but undeclared, how to fix it

Source: https://epubcc.com/epubcheck/opf-028/ (updated 2026-11-04)

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. Apart from the reserved prefixes, every prefix must be mapped to a vocabulary URI before it can be used.

- 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.
- The prefix attribute was removed or cleaned up while properties still use it.
- A typo in the prefix of a property, so it no longer matches the declared one.

## How to fix it

1. Read the prefix EPUBCheck names.
2. Add its mapping to the prefix attribute of the package element. Write each mapping as the prefix name, a colon, one space and the vocabulary URI, and separate mappings with whitespace, for example prefix="ibooks: http://vocabulary.itunes.apple.com/rdf/ibooks/vocabulary-extensions-1.0/ foaf: http://xmlns.com/foaf/spec/".
3. Or, if the property is not needed, remove it.

## When it's not this

If the prefix is declared but the declaration is badly written, that is a variant of OPF-004.

## Sources

- EPUB 3.3, §D.1.4 The prefix attribute: https://www.w3.org/TR/epub-33/#sec-prefix-attr (read 2026-11-04)
- EPUB 3.3, §D.1.5 Reserved prefixes: https://www.w3.org/TR/epub-33/#sec-reserved-prefixes (read 2026-11-04)

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