# RSC-020: invalid URL syntax in a link, how to fix it

Source: https://epubcc.com/epubcheck/rsc-020/ (updated 2026-11-04)

EPUBCheck RSC-020 is an error message: “"…" is not a valid URL (…).” A URL in the EPUB is not syntactically valid. Reading apps may fail to follow the link or load the resource, and the message gives the URL and the reason in brackets.

- 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.
- A typo in the scheme part of an absolute URL (a link whose scheme is well formed but not registered is HTM-025 instead).

## How to fix it

1. Find the URL named in the message.
2. Percent-encode spaces as %20 and other reserved characters, and use forward slashes.
3. Or rename the file so its URL needs no encoding, then validate again.

## When it's not this

If the URL is valid but the file it names is missing, that is RSC-007.

## Sources

- EPUB 3.3, §4.2.5 URLs in the OCF abstract container: https://www.w3.org/TR/epub-33/#sec-container-iri (read 2026-11-04)

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