Estoy tratando de usar el protocolo NSCoding en una clase que he escrito rápidamente, pero parece que no puedo entender por qué el compilador se queja de que "no se ajusta al protocolo NSCoding" cuando implemento los métodos requeridos:

class ServerInfo: NSObject, NSCoding {

    var username = ""
    var password = ""
    var domain = ""
    var location = ""
    var serverFQDN = ""
    var serverID = ""

    override init() {

    }

    init(coder aDecoder: NSCoder!) {
        self.username = aDecoder.decodeObjectForKey("username") as NSString
        self.password = aDecoder.decodeObjectForKey("password") as NSString
        self.domain = aDecoder.decodeObjectForKey("domain") as NSString
        self.location = aDecoder.decodeObjectForKey("location") as NSString
        self.serverFQDN = aDecoder.decodeObjectForKey("serverFQDN") as NSString
        self.serverID = aDecoder.decodeObjectForKey("serverID") as NSString
    }


    func encodeWithCoder(_aCoder: NSCoder!) {
        _aCoder.encodeObject(self.username, forKey: "username")
        _aCoder.encodeObject(self.password, forKey: "password")
        _aCoder.encodeObject(self.domain, forKey: "domain")
        _aCoder.encodeObject(self.location, forKey: "location")
        _aCoder.encodeObject(self.serverFQDN, forKey: "serverFQDN")
        _aCoder.encodeObject(self.serverID, forKey: "serverID")
    }

}

¿Es esto un error o solo me falta algo?