Message in an IRC client

compile-error-output.txt

Compilation completed with 2 errors and 0 warnings
error: overriding value arguments in class Message of type List[String];
value arguments needs `override' modifier
case class Ping(arguments: List[String]) extends Message(None, PING, arguments)
error: overriding value arguments in class Message of type List[String];
value arguments needs `override' modifier
case class Pong(arguments: List[String]) extends Message(None, PONG, arguments)

Message.scala

case class Message(origin: Option[Origin], command: NameAndFormat, arguments: List[String]) {

  def format = {
    val builder = new StringBuilder()
    origin.foreach(x => builder.append(":").append(x.name).append(" "))
    command match {
      case Status(c, n) => builder.append(c)
      case Command(n) => builder.append(n)
    }
    builder.append(" ")
    builder.append(arguments.mkString(" "))
    builder.append("\r\n")
    builder.toString
  }
}

case class Ping(arguments: List[String]) extends Message(None, PING, arguments)
case class Pong(arguments: List[String]) extends Message(None, PONG, arguments)