OPF-012 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-012`

Updated: Nov 3, 2026

EPUBCheck OPF-012 is an error message: “Item property "…" is not defined for media type "…".” A manifest item has a property that does not apply to its media type.

The official message

Item property "%1$s" is not defined for media type "%2$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

cover-image on a file that is not an image, nav on a file that is not XHTML, or svg/mathml/scripted on a file that is not a content document.

How to fix it

Move the property to the right item or remove it. EPUBCheck names the property and the media type.

Terms used here

Other OPF messages

OPF-011 itemref can’t have both page-spread-right and page-spread-left properties.
OPF-013 Resource "…" is declared with MIME type "…" in content, but has MIME type "…" in the package document.
OPF-014 The property "…" should be declared in the OPF file.
OPF-015 The property "…" should not be declared in the OPF file.
OPF-016 The element "rootfile" is missing its required attribute "full-path".
OPF-007 Re-declaration of reserved prefix "…".

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