Apex domains cannot have CNAME records.
Question and response
Question
dig nfv.gtt.net. NS
Answer
nfv.gtt.net. 4 NS ns6-build.gtt.net.
nfv.gtt.net. 4 NS ns5-build.gtt.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0