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

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

EPUBCheck OPF-004e is a warning message: “Invalid prefix declaration: found illegal whitespace between prefix and URI.” There is whitespace the syntax does not allow between a prefix and its URI.

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

## Typical causes

- Several spaces, a tab or a line break between the colon and the URI.
- 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/". Use exactly one space between the colon and the URI.
- Reserved prefixes such as dcterms, rendition and schema need no declaration at all.

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