# EPUBCheck CSS-002: Empty or NULL reference found

Source: https://epublab.com/epubcheck/css-002/ (updated 2026-11-03)

EPUBCheck CSS-002 is an error message: “Empty or NULL reference found.” The CSS has an empty reference, such as url() with nothing inside.

- 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.

## How to fix it

- Delete the declaration or fill in the URL.

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