Apex domains cannot have CNAME records.
Question and response
Question
dig bt. NS
Answer
bt. 5 NS ns1.druknet.bt.
bt. 5 NS auth61.ns.uu.net.
bt. 5 NS ns3.druknet.bt.
bt. 5 NS ns2.druknet.bt.
bt. 5 NS ns.itu.ch.
bt. 5 NS auth00.ns.uu.net.
bt. 5 NS phloem.uoregon.edu.
Authority
The response has no authority records.
Additional
ns.itu.ch. 5 A 156.106.192.121
ns1.druknet.bt. 168288 A 202.144.128.200
ns2.druknet.bt. 168288 A 202.144.128.210
ns3.druknet.bt. 160264 A 204.61.216.17
auth00.ns.uu.net. 5 A 198.6.1.65
phloem.uoregon.edu. 5 A 128.223.32.35
ns.itu.ch. 5 AAAA 2a00:7580:60:2141:0:0:0:10
ns1.druknet.bt. 168288 AAAA 2405:d000:0:100:0:0:0:200
ns2.druknet.bt. 168288 AAAA 2405:d000:0:100:0:0:0:210
ns3.druknet.bt. 160264 AAAA 2001:500:14:6017:ad:0:0:1
phloem.uoregon.edu. 5 AAAA 2001:468:d01:20:0:0:80df:2023
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0