Interpolation in url() causes malformed url error

[nkovacs]

This does not work:

http://sassmeister.com/gist/6ec224b93dceaf79bc7b

If I use a variable, it works:

http://sassmeister.com/gist/406bb98619e1107c4193

If I quote the url, it also works:

http://sassmeister.com/gist/9a5bf8575597335bd241

If I remove the dots, the leading / and the @, it also works:

http://sassmeister.com/gist/ea879c29b0c3c1a45ac4

[xzyfer]

it appears that interpolation with url() is mishandled. Simplified test cases FTR

foo {
  background-image: url(foo/#{"bar"}/baz.jpg);
}