# EPUBCheck OPF-050: TOC attribute references resource with…

Source: https://epublab.com/epubcheck/opf-050/ (updated 2026-11-03)

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.

- 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 or another file instead of the NCX.

## How to fix it

- Point toc at the NCX item (media type application/x-dtbncx+xml), or remove the attribute in an EPUB 3 without NCX.

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