Apex domains cannot have CNAME records.
Question and response
Question
dig onl. NS
Answer
onl. 5 NS c0.nic.onl.
onl. 5 NS a2.nic.onl.
onl. 5 NS a0.nic.onl.
onl. 5 NS b0.nic.onl.
Authority
The response has no authority records.
Additional
a0.nic.onl. 172800 A 65.22.136.1
a2.nic.onl. 172800 A 65.22.139.1
b0.nic.onl. 172800 A 65.22.137.1
c0.nic.onl. 172800 A 65.22.138.1
a0.nic.onl. 172800 AAAA 2a01:8840:86:0:0:0:0:1
a2.nic.onl. 172800 AAAA 2a01:8840:89:0:0:0:0:1
b0.nic.onl. 172800 AAAA 2a01:8840:87:0:0:0:0:1
c0.nic.onl. 172800 AAAA 2a01:8840:88:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0