css - Getting the error "The path is not of a legal form." using SquishIt -

i using squish render raw css page, using renderrawcontent..

however, getting error "the path not of legal form." stacktrace doesn't give useful information.

viewbag.rawcontent = squishit.framework.bundle.css() .add("~/assets/file1.css") .add("~/assets/css/file2.css") .forcerelease() .renderrawcontent("/assets/combined/files.css"); 

any ideas? have working on other projects (but not mvc), not sure what's wrong..