OPF-026 is reported as error by default. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.

EPUBCheck · Package document (OPF)

`OPF-026`

Updated: Nov 3, 2026

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.

The official message

Found malformed property value: "%1$s".

Default severity: Error. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.
EPUBCheck version: 5.4.0

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

Typical causes

Values such as "rendition:" or values with spaces or characters a property name cannot have.

How to fix it

Write the property as prefix:name (or a plain name from the default vocabulary) with nothing missing.

Terms used here

Other OPF messages

OPF-025 Property value list "…" is not allowed, only one value must be specified.
OPF-027 Undefined property: "…".
OPF-028 Undeclared prefix: "…".
OPF-029 The file "…" does not appear to match the media type …, as specified in the OPF file.
OPF-030 The unique-identifier "…" was not found.
OPF-021 Use of non-registered URI scheme type in href: "…".

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EPUBLab.