# EPUBCheck OPF-007: forbidden prefix mapping, how to fix it

Source: https://epubcc.com/epubcheck/opf-007/ (updated 2026-11-04)

EPUBCheck OPF-007 is a warning message: “Re-declaration of reserved prefix "…".” The prefix attribute declares a mapping that EPUB does not allow there, most often a re-declaration of a reserved prefix. EPUBCheck reports four related cases: OPF-007 (a warning) and OPF-007a, OPF-007b and OPF-007c (errors).

- Official message (EPUBCheck 5.4.0): Re-declaration of reserved prefix "%1$s".
- Default severity: Warning

## Typical causes

- OPF-007: a reserved prefix such as dcterms, rendition, schema or a11y is mapped to a URI other than its reserved one, usually because a prefix attribute was copied from another book.
- OPF-007a: the prefix "_" is declared, which EPUB forbids.
- OPF-007b: a prefix is mapped to the URI of a default vocabulary, whose terms must be used without a prefix.
- OPF-007c: a prefix is mapped to the Dublin Core /elements/1.1/ namespace, which belongs in an xmlns:dc declaration, not in the prefix attribute.

## How to fix it

1. Find the prefix attribute on the package element of the .opf file.
2. Delete the mapping EPUBCheck names: reserved prefixes work without being declared, default vocabularies take no prefix, and "_" cannot be declared.
3. Keep Dublin Core as xmlns:dc="http://purl.org/dc/elements/1.1/" on the package or metadata element.
4. If the attribute is now empty, remove it.

## When it's not this

If a prefix is used in a property but never declared, that is OPF-028; if the attribute's syntax is wrong, that is a variant of OPF-004.

## Sources

- EPUB 3.3, §D.1.5 Reserved prefixes: https://www.w3.org/TR/epub-33/#sec-reserved-prefixes (read 2026-11-04)
- EPUB 3.3, §D.1.3 Default vocabularies: https://www.w3.org/TR/epub-33/#sec-default-vocab (read 2026-11-04)

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