Apex domains cannot have CNAME records.
Question and response
Question
dig markmonitor.com. NS
Answer
markmonitor.com. 2 NS ns5.markmonitor.com.
markmonitor.com. 2 NS ns1.markmonitor.com.
markmonitor.com. 2 NS ns2.markmonitor.com.
markmonitor.com. 2 NS ha2.markmonitor.zone.
markmonitor.com. 2 NS ns4.markmonitor.com.
markmonitor.com. 2 NS ha4.markmonitor.zone.
markmonitor.com. 2 NS ns3.markmonitor.com.
markmonitor.com. 2 NS ns7.markmonitor.com.
markmonitor.com. 2 NS ns6.markmonitor.com.
Authority
The response has no authority records.
Additional
ha2.markmonitor.zone. 2 A 162.219.54.170
ha4.markmonitor.zone. 2 A 162.219.55.170
ns1.markmonitor.com. 2 A 149.112.160.1
ns2.markmonitor.com. 2 A 176.97.158.1
ns3.markmonitor.com. 2 A 149.112.161.1
ns4.markmonitor.com. 2 A 192.174.68.1
ns5.markmonitor.com. 2 A 149.112.160.2
ns6.markmonitor.com. 2 A 176.97.158.3
ns7.markmonitor.com. 2 A 149.112.161.2
ha2.markmonitor.zone. 2 AAAA 2620:10a:80eb:0:0:0:0:6b
ha4.markmonitor.zone. 2 AAAA 2620:10a:80ec:0:0:0:0:6b
ns1.markmonitor.com. 2 AAAA 2620:10a:80a8:0:0:0:0:1
ns2.markmonitor.com. 2 AAAA 2001:67c:10b8:0:0:0:0:1
ns3.markmonitor.com. 2 AAAA 2620:10a:80a9:0:0:0:0:1
ns4.markmonitor.com. 2 AAAA 2001:67c:1bc:0:0:0:0:1
ns5.markmonitor.com. 2 AAAA 2620:10a:80a8:0:0:0:0:2
ns6.markmonitor.com. 2 AAAA 2001:67c:10b8:0:0:0:0:3
ns7.markmonitor.com. 2 AAAA 2620:10a:80a9:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0