EPUB Error: Unique Identifier Not Found (OPF-048)

OPF-048: The "unique-identifier" attribute in the package element refers to an ID "bookid" that is not defined anywhere in the document

What causes this error?

OPF-048 fires when the unique-identifier attribute on the OPF <package> element names a cross-reference ID that doesn't match any <dc:identifier> element in the metadata block. This is not a missing ISBN — the identifier may be present and correctly formatted, but the attribute names used as the cross-reference don't agree. It usually happens when a template is edited on one side without updating the other.

How to fix it

  1. Open content.opf and find the opening <package> tag — note the exact value of its unique-identifier attribute (e.g., unique-identifier="bookid").
  2. In the <metadata> block, find all <dc:identifier> elements and check their id attributes (e.g., <dc:identifier id="isbn-13">978-...</dc:identifier>).
  3. Make the values match: either change the unique-identifier attribute to match an existing dc:identifier's id, or change the dc:identifier's id to match the unique-identifier value. Both sides must be identical.
  4. Confirm only one <dc:identifier> carries the id value that unique-identifier references.
  5. Re-validate with EPUBCheck and confirm OPF-048 is cleared.

Common questions

Is this the same as a missing ISBN?

No — a missing ISBN means the dc:identifier value is empty or the element is absent. OPF-048 means the identifier may be present and correctly formatted, but the package element's unique-identifier attribute points to an id that doesn't match any dc:identifier's id attribute. You can have a valid ISBN in the metadata and still get OPF-048.

Which platforms are strictest about this error?

Apple Books validates OPF-048 and rejects submissions with this error. KDP generally accepts files with this error since it can extract metadata without the cross-reference being intact, though metadata warnings may appear. EPUBCheck flags it at the Error level — fix before distributing regardless of platform.

Check your EPUB before uploading

The free EPUB Validator catches structure, metadata, and navigation errors — no signup, no Java, runs in the browser.

Validate Your EPUB Free →

Building your EPUB correctly from the start prevents most of these errors before they happen. See the EPUB formatting guide for the full workflow: manuscript cleanup, TOC, front matter, EPUB 3.0 generation, and validation.

Platform rejection guides: Why Apple Books rejects ebooks

Common EPUB errors: Empty title tag (Kobo metadata error)

Publishing checklists: Apple Books submission checklist