[Scala]ワーニングがとれない

package test

trait IHtmlNode {}

case class HtmlElement(val tagname:String) extends IHtmlNode {}

case class HtmlText(val text:String) extends IHtmlNode {}

case class Form(val tag:IHtmlNode){
    val (action:String, name:Option[String]) = tag match {
        case HtmlElement(tagname) =>    ("!", Some(tagname))
        case _ =>                       ("?", None)
    }
}

/*
Compiling 1 source file to /home/a-san/devel/scala/test/bin
 /home/a-san/devel/scala/test/src/test/Eliminated.scala:10: warning: 
non variable type-argument String in type pattern Option[String] is unchecked 
since it is eliminated by erasure
    val (action:String, name:Option[String]) = tag match {
                                  ^
 one warning found
 Compile succeeded with 1 warning; see the compiler output for details.
*/

2.9.1.final