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PKG-009: forbidden character in an EPUB file name

Plain English meaning

One of the files inside the EPUB has a name that EPUB rules do not allow. In the public sample experiment, the forbidden character was a vertical bar: |.

Why it happens

• A chapter, image, or internal file was named with a character that is unsafe inside EPUB packages.
• A conversion tool copied a title or source filename directly into the EPUB.
• The file was renamed manually, but references inside OPF, nav, or XHTML were not updated.

What you can check yourself

1. Rename a copy of the EPUB to .zip.
2. Look through the internal file list.
3. Check for unusual characters in file or folder names, especially characters that also cause trouble on Windows or in URLs.

What can usually be fixed

The mechanical fix is to rename the internal file and update every place that references it, including the OPF manifest, navigation document, and any XHTML links.

What cannot be promised

Renaming one file can reveal follow-on errors if the EPUB contains multiple broken links. The fix is still file-level, but the number of references matters.