Apex domains cannot have CNAME records.
Question and response
Question
dig BT.COM. NS
Answer
bt.com. 5 NS DYDNS0.BT.COM.
bt.com. 5 NS EDDNS0.BT.COM.
bt.com. 5 NS DYDNS1.BT.COM.
bt.com. 5 NS EDDNS1.BT.COM.
Authority
The response has no authority records.
Additional
dydns0.bt.com. 5 A 193.113.32.156
dydns1.bt.com. 5 A 193.113.32.157
eddns0.bt.com. 5 A 193.113.57.242
eddns1.bt.com. 5 A 193.113.57.243
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0