Parentheses Balancing in Scala

paren_balance.scala

def balance(chars: List[Char], open: Int): Boolean = {
  if (chars.isEmpty)
    open == 0
  else if (chars.head == '(')
    balance(chars.tail, open + 1)
  else if (chars.head == ')' && open > 0)
    // found an unmatched '(' earlier
    balance(chars.tail, open - 1)
  else
    balance(chars.tail, open)
}                                                 
  
val x = "(x + y + (z + 1)))"                    
balance(x.toList, 0)
  
val y = "())("
balance(y.toList, 0)

It is not covering below example:
"())"