Creating subcommands does not work about clikt HOT 2 CLOSED

import com.github.ajalt.clikt.core.CliktCommand
import com.github.ajalt.clikt.core.subcommands

class Database: CliktCommand() {
    override fun run() = Unit
}

class Init: CliktCommand(help="Initialize the database") {
    override fun run() {
        echo("Initialized the database.")
    }
}

class Drop: CliktCommand(help="Drop the database") {
    override fun run() {
        echo("Dropped the database.")
    }
}

fun main(args: Array<String>) = Database()
        .subcommands(Init(), Drop())
        .main(args)