Apex domains cannot have CNAME records.
Question and response
Question
dig htdb.jp. NS
Answer
htdb.jp. 5 NS ns-479.awsdns-59.com.
htdb.jp. 5 NS ns-1311.awsdns-35.org.
htdb.jp. 5 NS ns-1001.awsdns-61.net.
htdb.jp. 5 NS ns-1728.awsdns-24.co.uk.
Authority
The response has no authority records.
Additional
ns-479.awsdns-59.com. 1 A 205.251.193.223
ns-1001.awsdns-61.net. 1 A 205.251.195.233
ns-1311.awsdns-35.org. 1 A 205.251.197.31
ns-1728.awsdns-24.co.uk. 1 A 205.251.198.192
ns-479.awsdns-59.com. 1 AAAA 2600:9000:5301:df00:0:0:0:1
ns-1001.awsdns-61.net. 1 AAAA 2600:9000:5303:e900:0:0:0:1
ns-1311.awsdns-35.org. 1 AAAA 2600:9000:5305:1f00:0:0:0:1
ns-1728.awsdns-24.co.uk. 1 AAAA 2600:9000:5306:c000:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0