# EPUBCheck OPF-027: property not in vocabulary, how to fix it

Source: https://epubcc.com/epubcheck/opf-027/ (updated 2026-11-04)

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. EPUBCheck knows the EPUB vocabularies and checks every unprefixed or reserved-prefix property against them.

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

## Typical causes

- A misspelling, such as properties="cover" instead of "cover-image".
- An invented property name used for a tool's own purposes.
- A property from a newer or older version of a vocabulary that EPUBCheck does not recognise.

## How to fix it

1. Read the property EPUBCheck quotes.
2. Look it up in the EPUB 3.3 vocabularies and correct the spelling if it exists.
3. If it is a custom property, put it under your own prefix declared in the prefix attribute, or remove it.

## When it's not this

If the property uses a prefix that is not declared, EPUBCheck reports OPF-028 instead.

## Sources

- EPUB 3.3, §D.1.2 The property data type: https://www.w3.org/TR/epub-33/#sec-property-datatype (read 2026-11-04)

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