# EPUBCheck OPF-006: Invalid prefix declaration: URI … is…

Source: https://epublab.com/epubcheck/opf-006/ (updated 2026-11-03)

EPUBCheck OPF-006 is an error message: “Invalid prefix declaration: URI "…" is not a valid URI.” The URI given for a prefix is not a valid URI.

- Official message (EPUBCheck 5.4.0): Invalid prefix declaration: URI "%1$s" is not a valid URI.
- Default severity: Error

## Typical causes

- A typo in the URI, or text that is not a URI where the URI 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/". Correct the URI to the vocabulary's published address.
- Reserved prefixes such as dcterms, rendition and schema need no declaration at all.

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