Apex domains cannot have CNAME records.
Question and response
Question
dig bing.com. NS
Answer
bing.com. 5 NS ns4-204.azure-dns.info.
bing.com. 5 NS ns1-204.azure-dns.com.
bing.com. 5 NS dns4.p09.nsone.net.
bing.com. 5 NS ns2-204.azure-dns.net.
bing.com. 5 NS dns1.p09.nsone.net.
bing.com. 5 NS ns3-204.azure-dns.org.
bing.com. 5 NS dns2.p09.nsone.net.
bing.com. 5 NS dns3.p09.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0