Apex domains cannot have CNAME records.
Question and response
Question
dig constant.com. NS
Answer
constant.com. 5 NS ns2.constant.com.
constant.com. 5 NS ns1.constant.com.
Authority
The response has no authority records.
Additional
ns1.constant.com. 5 A 68.232.191.191
ns2.constant.com. 5 A 68.232.191.232
ns1.constant.com. 5 AAAA 2001:19f0:ccc:0:0:0:0:1
ns2.constant.com. 5 AAAA 2001:19f0:ccc:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0