Apex domains cannot have CNAME records.
Question and response
Question
dig dnb.com. NS
Answer
dnb.com. 4 NS a6-66.akam.net.
dnb.com. 4 NS a12-66.akam.net.
dnb.com. 4 NS a13-66.akam.net.
dnb.com. 4 NS a1-67.akam.net.
dnb.com. 4 NS a7-66.akam.net.
dnb.com. 4 NS a10-67.akam.net.
Authority
The response has no authority records.
Additional
a1-67.akam.net. 4 A 193.108.91.67
a1-67.akam.net. 4 AAAA 2600:1401:2:0:0:0:0:43
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0