# EPUBCheck OPF-004d: Invalid prefix declaration: prefix …

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

EPUBCheck OPF-004d is an error message: “Invalid prefix declaration: prefix "…" must be separated by its URI with a space.” A prefix in the prefix attribute is not separated from its URI by a space.

- Official message (EPUBCheck 5.4.0): Invalid prefix declaration: prefix "%1$s" must be separated by its URI with a space.
- Default severity: Error

## Typical causes

- A mapping written as "ibooks:http://…" with no space after the colon.
- 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/". Put 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.