# EPUBCheck OPF-021: unregistered URI scheme, how to fix it

Source: https://epubcc.com/epubcheck/opf-021/ (updated 2026-11-04)

EPUBCheck OPF-021 is a warning message: “Use of non-registered URI scheme type in href: "…".” An href uses a URI scheme that is not registered, unlike http, https or mailto. Reading apps may not know what to do with an unregistered scheme, so the link can fail silently.

- Official message (EPUBCheck 5.4.0): Use of non-registered URI scheme type in href: "%1$s".
- Default severity: Warning

## Typical causes

- A typo in the scheme, such as htp:// or http//.
- An app-specific scheme that only works in one program, such as a link meant for a particular reader.
- A local file path (C:\…) written where a URL was expected.

## How to fix it

1. Find the href EPUBCheck names.
2. Correct the scheme to a standard one (https, http, mailto…) if it is a typo.
3. If the link only works in one app or on one computer, remove it or replace it with a web address.

## When it's not this

If the link points to a file inside the EPUB that does not exist, that is RSC-007.

## Sources

- EPUB 3.3, §5.3.2 The href attribute: https://www.w3.org/TR/epub-33/#attrdef-href (read 2026-11-04)

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