Apex domains cannot have CNAME records.
Question and response
Question
dig apidocpdf.ivcert.net. NS
Answer
apidocpdf.ivcert.net. 5 NS ns-272.awsdns-34.com.
apidocpdf.ivcert.net. 5 NS ns-582.awsdns-08.net.
apidocpdf.ivcert.net. 5 NS ns-1474.awsdns-56.org.
apidocpdf.ivcert.net. 5 NS ns-1773.awsdns-29.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0