kotlin エラー「error: cannot create an instance of an abstract class」の解決方法

abstract class Oya() {

    abstract fun f()    

}

class Ko() : Oya() {

    override fun f() { println("hello") }

}

fun main(){

    val oya = Oya()
    oya.f()

}

error: cannot create an instance of an abstract class
    val oya = Oya()

abstract class Oya() {

    abstract fun f()    

}

class Ko() : Oya() {

    override fun f() { println("hello") }

}

fun main(){

    val ko = Ko()
    ko.f() // hello

}