diff --git a/se/se_epub_lint.py b/se/se_epub_lint.py
index ce999f23..658fe2a7 100644
--- a/se/se_epub_lint.py
+++ b/se/se_epub_lint.py
@@ -452,6 +452,7 @@
"x-018", "Unused [xhtml]id[/] attribute."
"x-019", "Unexpected value of [attr]id[/] attribute. Expected: [attr]{unexpected_id[1]}[/]."
"x-020", "Link to [path][link=file://{local_css_path}]se.css[/][/] in [xhtml]
[/], but not an SE boilerplate file."
+"x-021", "[xhtml]