Apex domains cannot have CNAME records.
Question and response
Question
dig iso.afrinic.net. NS
Answer
iso.afrinic.net. 5 NS ns4.afrinic.net.
iso.afrinic.net. 5 NS ns1.afrinic.net.
iso.afrinic.net. 5 NS ns2.afrinic.net.
Authority
The response has no authority records.
Additional
ns1.afrinic.net. 5 A 196.216.2.1
ns2.afrinic.net. 5 A 196.216.168.10
ns1.afrinic.net. 5 AAAA 2001:42d0:0:0:0:200:2:1
ns2.afrinic.net. 5 AAAA 2001:43f8:120:0:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0