diff --git a/lib/CSS/Grammar/CSS21.rakumod b/lib/CSS/Grammar/CSS21.rakumod
index 0c6a009..0158fd7 100644
--- a/lib/CSS/Grammar/CSS21.rakumod
+++ b/lib/CSS/Grammar/CSS21.rakumod
@@ -93,7 +93,7 @@ rule pseudo:sym<function> {':'[<pseudo-function>||<pseudo-function=.any-pseudo-f
 # assume anything else is a class
 rule pseudo:sym<class>    {':' <class=.Ident><!before '('>}
 
-rule any-function         {<Ident>'(' [ <expr>? || <any-args> ] ')'}
+rule any-function         {<Ident>'(' [ <expr>? <?before ')'> || <any-args> ] ')'}
 
 proto rule pseudo-function {*}
 rule pseudo-function:sym<lang> {:i'lang(' [ <Ident> || <any-args> ] ')'}