Not found exception for public assets

I'm trying to combine some css in the server's /public directory.

I used the following concat group: 

``` scala
Concat.groups := Seq(
      "styles.css" -> group{
        val x = (resourceDirectory.in(Assets).value / "css") * "*.css"
        println(x.absString)
        x
      }
    ),
```

This prints the correct files, but I receive not-found warnings. 
