Parse error in at-rule prelude with non-quoted URL

[racodond]

Parsing OK:

@document url("http://www.w3.org/"),
          url-prefix("http://www.w3.org/"),
          domain(mozilla.org),
          regexp("https:.*")
{
  body {
    color: purple;
    background: yellow;
  }
}

Removing double quotes around url-prefix("http://www.w3.org/"), parsing KO.