Apex domains cannot have CNAME records.
Question and response
Question
dig bgp.tools. NS
Answer
bgp.tools. 4 NS ns-1799.awsdns-32.co.uk.
bgp.tools. 4 NS ns2-35.azure-dns.net.
bgp.tools. 4 NS ns1-35.azure-dns.com.
bgp.tools. 4 NS ns1.exoscale.io.
bgp.tools. 4 NS ns-1329.awsdns-38.org.
bgp.tools. 4 NS ns1.exoscale.net.
bgp.tools. 4 NS ns3-35.azure-dns.org.
bgp.tools. 4 NS ns-721.awsdns-26.net.
bgp.tools. 4 NS backup-ns.bgp.tools.
bgp.tools. 4 NS ns1.exoscale.ch.
bgp.tools. 4 NS ns-302.awsdns-37.com.
bgp.tools. 4 NS ns4-35.azure-dns.info.
bgp.tools. 4 NS ns1.exoscale.com.
Authority
The response has no authority records.
Additional
backup-ns.bgp.tools. 4 A 185.230.223.156
backup-ns.bgp.tools. 4 AAAA 2a0c:2f07:9459:0:0:0:0:aa53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0