PKG-001 is reported as warning by default. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.

EPUBCheck · Package and ZIP container

`PKG-001`

Updated: Nov 3, 2026

EPUBCheck PKG-001 is a warning message: “Validating the EPUB against version … but detected version ….” EPUBCheck was told to validate one EPUB version but the package document declares another.

The official message

Validating the EPUB against version %1$s but detected version %2$s.

Default severity: Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
EPUBCheck version: 5.4.0

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

Typical causes

EPUBCheck was run with -v 2.0 or -v 3.0 and the file's package element has a different version attribute.

How to fix it

Run EPUBCheck without -v on a packaged .epub file: it reads the version from the package document.
If the version attribute itself is wrong, set it to the version the content really follows: version="3.0" for EPUB 3.

Terms used here

Other PKG messages

PKG-003 Unable to read EPUB file header. This is likely a corrupted EPUB file.
PKG-004 Corrupted EPUB ZIP header.
PKG-005 The mimetype file has an extra field of length …. The use of the extra field feature of the ZIP format is not permitted for the mimetype file.
PKG-006 Mimetype file entry is missing or is not the first file in the archive.
PKG-007 The content of the mimetype file must be the string "application/epub+zip"…
PKG-009 The file name "…" contains characters that are not allowed in OCF file names: ….

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