func return does notReturn String from my conform to protocol StringLiteralConvertible error

I have class which has the func of return path to some file:

func getArchivePathFor(fileName: String!) -> String {
        var paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String     
        var path: String = paths.stringByAppendingPathComponent(fileName!)
        return path    
    }

But when I try to use it in my code like:

let val = Helpers.getArchivePathFor("MyList.plist")

I get the error: "Return String from my func return does not conform to protocol StringLiteralConvertible error". Why so?