Apex domains cannot have CNAME records.
Question and response
Question
dig bt.systems. NS
Answer
bt.systems. 5 NS ns3.bt.systems.
bt.systems. 5 NS ns1.bt.systems.
bt.systems. 5 NS ns2.bt.systems.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0