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

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

EPUBCheck OPF-004f is a warning message: “Invalid prefix declaration: found illegal whitespace between prefix mappings.” There is whitespace the syntax does not allow between two prefix mappings.

- Official message (EPUBCheck 5.4.0): Invalid prefix declaration: found illegal whitespace between prefix mappings.
- Default severity: Warning

## Typical causes

- Mappings separated by something other than plain whitespace, or written without any separator.
- 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/". Separate each prefix-URI pair from the next with whitespace.
- Reserved prefixes such as dcterms, rendition and schema need no declaration at all.

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