# EPUBCheck OPF-050: spine toc is not an NCX, how to fix it

Source: https://epubcc.com/epubcheck/opf-050/ (updated 2026-11-04)

EPUBCheck OPF-050 is an error message: “TOC attribute references resource with non-NCX mime type; "application/x-dtbncx+xml" is expected.” The spine's toc attribute points to an item that is not an NCX file. In EPUB 2 (and in EPUB 3 books that keep an NCX for old readers) toc must name the NCX item.

- Official message (EPUBCheck 5.4.0): TOC attribute references resource with non-NCX mime type; "application/x-dtbncx+xml" is expected.
- Default severity: Error

## Typical causes

- toc set to the id of the navigation document instead of the NCX.
- toc pointing at a chapter or another file by mistake.
- The NCX declared with the wrong media type, so it does not look like an NCX.

## How to fix it

1. Find the NCX item in the manifest; its media type must be application/x-dtbncx+xml.
2. Set the spine's toc attribute to that item's id.
3. In an EPUB 3 without an NCX, remove the toc attribute.

## When it's not this

If toc names an id that does not exist at all, that is OPF-049.

## Sources

- EPUB 3.3, §5.7.1 The spine element: https://www.w3.org/TR/epub-33/#sec-spine-elem (read 2026-11-04)
- EPUB 3.3, §5.9.5 NCX: https://www.w3.org/TR/epub-33/#sec-opf2-ncx (read 2026-11-04)

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