Kotlin's lambda as a last parameter + varargs + higher order function

LastParameterLambda.kt

typealias Operator = (Int, Int) -> Int
fun reduce(zero: Int, vararg xs: Int): ((Operator) -> Int) {
  var acc = zero
  fun reductor(op: Operator): Int {
    xs.forEach { acc =  op(acc, it)}
    return acc
  }
  return { reductor(it) }
}

/**
* According to https://kotlinlang.org/docs/reference/lambdas.html#passing-a-lambda-to-the-last-parameter
* we should be able to omit the parentheses, but the `vargargs` seem to confuse the parser.
*
* Is this expected behaviour?
*/

val ok = reduce(1, 2, 3, 4)({ x, y -> x + y })
val alsoOk = (reduce(1, 2, 3, 4)) { x, y -> x + y }
val notOk = reduce(1, 2, 3, 4) { x, y -> x + y } // Error:(13, 31) Passing value as a vararg is only allowed inside a parenthesized argument list