Apex domains cannot have CNAME records.
Question and response
Question
dig od1.vtiger.com. NS
Answer
od1.vtiger.com. 5 NS ns-1441.awsdns-52.org.
od1.vtiger.com. 5 NS ns-692.awsdns-22.net.
od1.vtiger.com. 5 NS ns-298.awsdns-37.com.
od1.vtiger.com. 5 NS ns-1878.awsdns-42.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0