ns1.bt.systems has no CNAME records.
Question and response
Question
dig ns1.bt.systems. NS
Answer
The response has no answer records.
Authority
bt.systems. 358 SOA ns1.bt.systems. dns.bt.systems. 2025081901 86400 7200 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0