Apex domains cannot have CNAME records.
Question and response
Question
dig od2.vtiger.com. NS
Answer
od2.vtiger.com. 5 NS ns-2003.awsdns-58.co.uk.
od2.vtiger.com. 5 NS ns-445.awsdns-55.com.
od2.vtiger.com. 5 NS ns-685.awsdns-21.net.
od2.vtiger.com. 5 NS ns-1426.awsdns-50.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0