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EPUBCheck · Package and ZIP container

PKG-016

Updated:

EPUBCheck PKG-016 is a warning message: “Use only lowercase characters for the EPUB file extension for maximum compatibility.” The file extension is written in uppercase (.EPUB); lowercase .epub is the most compatible.

The official message

Use only lowercase characters for the EPUB file extension for maximum compatibility.

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 or renamed on a system or tool that uppercased the extension.

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-017 Uncommon EPUB file extension.
  • PKG-014 The EPUB contains empty directory "…".
  • PKG-012 The file name "…" contains non-ASCII characters, which might create interoperability issues with older reading systems.
  • PKG-011 The file name "…" is not allowed to end with ".".
  • PKG-021 Corrupted image file encountered.
  • PKG-010 The file name "…" contains spaces, which may create interoperability issues with older reading systems.

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

FAQ

What does EPUBCheck PKG-016 mean?

EPUBCheck PKG-016 is a warning message: “Use only lowercase characters for the EPUB file extension for maximum compatibility.” The file extension is written in uppercase (.EPUB); lowercase .epub is the most compatible.

Is PKG-016 an error?

PKG-016 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.