OPF-006 is reported as error by default. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.

EPUBCheck · Package document (OPF)

`OPF-006`

Updated: Nov 3, 2026

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.

The official message

Invalid prefix declaration: URI "%1$s" is not a valid URI.

Default severity: Error. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.
EPUBCheck version: 5.4.0

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

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.

Terms used here

Other OPF messages

OPF-005 Invalid prefix declaration: URI for prefix "…" doesn’t exist.
OPF-007 Re-declaration of reserved prefix "…".
OPF-004 Invalid prefix declaration: leading or trailing whitespace is not allowed.
OPF-004a Invalid prefix declaration: found empty prefix.
OPF-004b Invalid prefix "…": must be a valid non-colonized name (aka "NCName").
OPF-004c Invalid prefix declaration: prefix "…" must be immediately followed by a colon character (":").

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EPUBLab.