# CSS-002: empty url() reference in CSS, how to fix it

Source: https://epubcc.com/epubcheck/css-002/ (updated 2026-11-04)

EPUBCheck CSS-002 is an error message: “Empty or NULL reference found.” The CSS has an empty reference, such as url() with nothing inside. An empty reference points nowhere, so reading apps cannot load anything from it, and EPUBCheck reports it as an error.

- Official message (EPUBCheck 5.4.0): Empty or NULL reference found.
- Default severity: Error

## Typical causes

- A background or font URL removed without removing the declaration.
- A stylesheet generated from a template whose image or font path was never filled in.

## How to fix it

1. Open the file EPUBCheck names (the message gives the path, line and column) in a text or EPUB editor such as Sigil, or unzip the EPUB and open it there.
2. Delete the declaration with the empty url(), or fill in the path to a file that is in the EPUB.
3. If you add a file, list it in the manifest with its media type.
4. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

## When it's not this

If url() names a file that is not in the EPUB, the message is RSC-007.

## Sources

- EPUBCheck 5.4.0 message texts (MessageBundle.properties, w3c/epubcheck on GitHub): https://github.com/w3c/epubcheck/blob/v5.4.0/src/main/resources/com/adobe/epubcheck/messages/MessageBundle.properties (read 2026-11-04)

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