@reference not finding assets

The `@reference` directive doesn't seem to be working. Here's my LESS files:

Content/Styles/main.less:

``` less
// @reference variables.less
body { color: red; }
```

Content/Styles/variables.less:

``` less
@link: #8cbf33;
@linkHover: darken(@link, 15%);
@conversationLink: #60a800;
@conversationHover: #608323;
```

I know in LESS you can use `@import` but there was a use case where I needed to `@reference` a normal CSS file due to vendor specific viewport styles that were causing the LESS compiler to complain (as well as having to reference the Bootstrap compiled CSS files). However, this `@reference` issue seems to affect any file type referenced.

In my Cassette configuration script I have the following:

``` csharp
bundles.Add("~/Bundles/styles", new []{
"~/Content/Styles/main.less"
});
```