Apex domains cannot have CNAME records.
Question and response
Question
dig linkedin.com. NS
Answer
linkedin.com. 5 NS ns1-42.azure-dns.com.
linkedin.com. 5 NS ns4-42.azure-dns.info.
linkedin.com. 5 NS dns1.p09.nsone.net.
linkedin.com. 5 NS ns2-42.azure-dns.net.
linkedin.com. 5 NS dns3.p09.nsone.net.
linkedin.com. 5 NS dns2.p09.nsone.net.
linkedin.com. 5 NS ns3-42.azure-dns.org.
linkedin.com. 5 NS dns4.p09.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0