# EPUBCheck OPF-027: Undefined property: …

Source: https://epublab.com/epubcheck/opf-027/ (updated 2026-11-03)

EPUBCheck OPF-027 is an error message: “Undefined property: "…".” A property is not defined in the vocabulary it belongs to, so reading apps will not understand it.

- Official message (EPUBCheck 5.4.0): Undefined property: "%1$s".
- Default severity: Error

## Typical causes

- A misspelling or an invented name, such as properties="cover" instead of "cover-image".

## How to fix it

- Use a property from the EPUB vocabulary, or put a custom property under a declared prefix.

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