Apex domains cannot have CNAME records.
Question and response
Question
dig gov. NS
Answer
gov. 5 NS d.ns.gov.
gov. 5 NS b.ns.gov.
gov. 5 NS a.ns.gov.
gov. 5 NS c.ns.gov.
Authority
The response has no authority records.
Additional
a.ns.gov. 64433 A 199.33.230.1
b.ns.gov. 64433 A 199.33.231.1
c.ns.gov. 66735 A 199.33.232.1
d.ns.gov. 49880 A 199.33.233.1
a.ns.gov. 64433 AAAA 2001:503:ff40:0:0:0:0:1
b.ns.gov. 64433 AAAA 2001:503:ff41:0:0:0:0:1
c.ns.gov. 66735 AAAA 2001:503:ff42:0:0:0:0:1
d.ns.gov. 49880 AAAA 2001:503:ff43:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0