Apex domains cannot have CNAME records.
Question and response
Question
dig isi.deterlab.net. NS
Answer
isi.deterlab.net. 5 NS ns2.deterlab.net.
isi.deterlab.net. 5 NS ns1.deterlab.net.
Authority
The response has no authority records.
Additional
ns1.deterlab.net. 5 A 206.117.25.50
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0