问Scala编译报错：SLS 8.5，请解释下原因？

object Main extends App {
  var tokens = Array("10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+")
  println(Solution.evalRPN(tokens))
}

object Solution {
  def evalRPN(tokens: Array[String]): Int = {
    val stack = new scala.collection.mutable.Stack[Int]

    tokens.foreach(token =>
      token matches {
        case "+" => stack.push(stack.pop() + stack.pop())
        case "-" => stack.push(-stack.pop() + stack.pop())
        case "*" => stack.push(stack.pop() * stack.pop())
        case "/" => {
          val top = stack.pop();
          stack.push(stack.pop() / top)
        }
        case _ => stack.push(stack.pop())
      })

    stack.pop()
  }
}