swc-project/swc · error · Error

Expected '--' at the start of custom property name

Error message

Expected '--' at the start of custom property name

What it means

Fired while parsing a CustomPropertyName: an Ident token was present, but its value did not start with `--`, which the CSS custom properties spec requires for custom property names. Input at fault is a declaration whose property looks like a custom property context but has a normal name, e.g. using a non-`--` prefixed name where --var syntax is mandatory.

Source

Thrown at crates/swc_css_parser/src/parser/values_and_units/mod.rs:1907

        }
    }
}

impl<I> Parse<CustomPropertyName> for Parser<I>
where
    I: ParserInput,
{
    fn parse(&mut self) -> PResult<CustomPropertyName> {
        let span = self.input.cur_span();

        if !is!(self, Ident) {
            return Err(Error::new(span, ErrorKind::Expected("Ident")));
        }

        match bump!(self) {
            Token::Ident { value, raw, .. } => {
                if !value.starts_with("--") {
                    return Err(Error::new(
                        span,
                        ErrorKind::Expected("'--' at the start of custom property name"),
                    ));
                } else if &*value == "--" {
                    return Err(Error::new(
                        span,
                        ErrorKind::Expected("valid dashed, '--' is not valid custom property name"),
                    ));
                }

                Ok(CustomPropertyName {
                    span,
                    value,
                    raw: Some(raw),
                })
            }
            _ => {
                unreachable!()

View on GitHub (pinned to 5176682b65)

Solutions

1. Prefix the property name with '--' (e.g. '--brand-color')
2. Use a normal declaration if a standard property was intended

When it happens

Trigger: Thrown at crates/swc_css_parser/src/parser/values_and_units/mod.rs:1907 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/c46c246dd10ada72. Report an issue: GitHub.