# EPUBCheck PKG-017: Uncommon EPUB file extension

Source: https://epublab.com/epubcheck/pkg-017/ (updated 2026-11-03)

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.

- Official message (EPUBCheck 5.4.0): Uncommon EPUB file extension.
- Default severity: Warning
- EPUBCheck's suggestion: For maximum compatibility, use ".epub".

## 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.

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