# EPUBCheck OPF-004a: Invalid prefix declaration: found…

Source: https://epublab.com/epubcheck/opf-004a/ (updated 2026-11-03)

EPUBCheck OPF-004a is an error message: “Invalid prefix declaration: found empty prefix.” The prefix attribute contains an empty prefix (a colon with no name before it).

- Official message (EPUBCheck 5.4.0): Invalid prefix declaration: found empty prefix.
- Default severity: Error

## Typical causes

- A stray colon, or a URI written where the prefix name should be.
- Hand-edited or tool-generated package documents that write the prefix attribute with the wrong separators.

## 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/". Give every mapping a name before the colon.
- Reserved prefixes such as dcterms, rendition and schema need no declaration at all.

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