# EPUBCheck OPF-026: malformed property value, how to fix it

Source: https://epubcc.com/epubcheck/opf-026/ (updated 2026-11-04)

EPUBCheck OPF-026 is an error message: “Found malformed property value: "…".” A property value is malformed, for example a prefix with nothing after the colon. Property values must be a name, optionally preceded by a prefix and a colon, and nothing else.

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

## Typical causes

- Values such as "rendition:" with the name missing.
- Values with spaces or characters that a property name cannot contain.
- A full URI pasted where a short prefixed name was expected.

## How to fix it

1. Find the value EPUBCheck quotes in the message.
2. Rewrite it as prefix:name (for example rendition:layout) or as a plain name from the default vocabulary.
3. If the value has a prefix, check that the prefix is reserved or declared in the prefix attribute.

## When it's not this

If the value is well formed but unknown, that is OPF-027; if its prefix is undeclared, OPF-028.

## 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.