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EPUBCheck · Package and ZIP container

PKG-017

Updated:

EPUBCheck PKG-017 is a warning message: “Uncommon EPUB file extension.” The file does not end in .epub, so some reading apps and stores may not recognise it.

The official message

Uncommon EPUB file extension.

EPUBCheck's suggestion: For maximum compatibility, use ".epub".

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

Typical causes

  • A file saved with another extension, such as .zip or .epub3, or none.

How to fix it

  • Rename the file to end in .epub. Nothing inside the EPUB needs to change.

Terms used here

Other PKG messages

  • PKG-016 Use only lowercase characters for the EPUB file extension for maximum compatibility.
  • PKG-014 The EPUB contains empty directory "…".
  • PKG-021 Corrupted image file encountered.
  • PKG-012 The file name "…" contains non-ASCII characters, which might create interoperability issues with older reading systems.
  • PKG-022 Wrong file extension for image. The image is a "…" file but has the file extension "…".
  • PKG-011 The file name "…" is not allowed to end with ".".

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

FAQ

What does EPUBCheck PKG-017 mean?

EPUBCheck PKG-017 is a warning message: “Uncommon EPUB file extension.” The file does not end in .epub, so some reading apps and stores may not recognise it.

Is PKG-017 an error?

PKG-017 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.