Apex domains cannot have CNAME records.
Question and response
Question
dig umbler.com. NS
Answer
umbler.com. 5 NS ns2.umbler.com.
umbler.com. 5 NS ns3.umbler.com.
umbler.com. 5 NS ns4.umbler.com.
umbler.com. 5 NS ns1.umbler.com.
Authority
The response has no authority records.
Additional
ns1.umbler.com. 5 A 40.121.88.85
ns2.umbler.com. 5 A 177.55.96.6
ns3.umbler.com. 5 A 177.55.116.14
ns4.umbler.com. 5 A 177.55.96.6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0