diff --git a/lib/CSS/Module/CSS3/Namespaces.rakumod b/lib/CSS/Module/CSS3/Namespaces.rakumod
index af6aa8f8..a6e36672 100644
--- a/lib/CSS/Module/CSS3/Namespaces.rakumod
+++ b/lib/CSS/Module/CSS3/Namespaces.rakumod
@@ -6,7 +6,7 @@ use v6;
 
 unit class CSS::Module::CSS3::Namespaces;
 
-rule at-decl:sym<namespace> {'@'(:i'namespace') <ns-prefix=.Ident>? [<url=.url-string>|<url>] ';' }
+rule at-decl:sym<namespace> {'@'(:i'namespace') <ns-prefix=.Ident>?! [<url=.url-string>|<url>] ';' }
 
 class Actions {
     use CSS::Grammar::Defs :CSSObject;