# EPUBCheck RSC-020: … is not a valid URL (…)

Source: https://epublab.com/epubcheck/rsc-020/ (updated 2026-11-03)

EPUBCheck RSC-020 is an error message: “"…" is not a valid URL (…).” A URL in the EPUB is not syntactically valid.

- Official message (EPUBCheck 5.4.0): "%1$s" is not a valid URL (%2$s)
- Default severity: Error

## Typical causes

- Unencoded spaces or other characters that must be percent-encoded in a URL.
- Backslashes used as path separators (a Windows habit) instead of forward slashes.

## How to fix it

- Percent-encode spaces as %20 and other reserved characters, use forward slashes, or rename the file so its URL needs no encoding.

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