# EPUBCheck NCX-001: NCX uid does not match OPF, how to fix it

Source: https://epubcc.com/epubcheck/ncx-001/ (updated 2026-11-04)

EPUBCheck NCX-001 is an error message: “NCX identifier ("…") does not match OPF identifier ("…").” The NCX's dtb:uid does not match the package's unique identifier. The NCX is the EPUB 2 table of contents, and its dtb:uid must repeat the identifier the package document names in unique-identifier, or some older reading systems may reject the file.

- Official message (EPUBCheck 5.4.0): NCX identifier ("%1$s") does not match OPF identifier ("%2$s").
- Default severity: Error

## Typical causes

- The book's identifier was changed in the package document but not in the NCX.
- An NCX copied from another book or from a template.

## How to fix it

1. Find the dc:identifier whose id is named by the unique-identifier attribute of the package element.
2. Copy its value exactly into the NCX: <meta name="dtb:uid" content="..."/>.
3. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

## When it's not this

If the value matches but has spaces or line breaks around it, the note is NCX-004.

## Sources

- EPUB 3.3, §5.9.5 NCX: https://www.w3.org/TR/epub-33/#sec-opf2-ncx (read 2026-11-04)
- EPUB 3.3, §5.5.3.1.1 The dc:identifier element: https://www.w3.org/TR/epub-33/#sec-opf-dcidentifier (read 2026-11-04)

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