# EPUBCheck OPF-021: Use of non-registered URI scheme type…

Source: https://epublab.com/epubcheck/opf-021/ (updated 2026-11-03)

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 (not http, https, mailto and other standard schemes).

- 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//, or an app-specific scheme.

## How to fix it

- Correct the scheme, or remove the link if it only works in one app.

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