Non-exhaustive pattern match warning

Browser.scala

package com.joescii.sbtjs

import com.gargoylesoftware.htmlunit. { BrowserVersion => HUBrowserVersion }
import HUBrowserVersion._

sealed trait Browser

object Browsers extends Browsers
trait Browsers {
  case object Firefox38 extends Browser
  case object InternetExplorer8 extends Browser
  case object InternetExplorer11 extends Browser
  case object Chrome extends Browser
  case object Edge extends Browser
}

private [sbtjs] object BrowserVersion {
  import Browsers._

  def apply(b:Browser):HUBrowserVersion = b match {
    case Firefox38 => FIREFOX_38
    case InternetExplorer8 => INTERNET_EXPLORER_8
    case InternetExplorer11 => INTERNET_EXPLORER_11
    case Chrome => CHROME
    case Edge => EDGE
  }
}

compile.txt

[warn] C:\code\sbt-js-test\src\main\scala\com\joescii\sbtjs\Browser.scala:20: match may not be exhaustive.
[warn] It would fail on the following inputs: Chrome, Edge, Firefox38, InternetExplorer11, InternetExplorer8
[warn]   def apply(b:Browser):HUBrowserVersion = b match {
[warn]                                           ^