Apex domains cannot have CNAME records.
Question and response
Question
dig smartdomains.IN. NS
Answer
smartdomains.IN. 5 NS v3a2.smartdomains.IN.
smartdomains.IN. 5 NS info349658.mars.orderbox-dns.com.
smartdomains.IN. 5 NS info349658.mercury.orderbox-dns.com.
smartdomains.IN. 5 NS info349658.venus.orderbox-dns.com.
smartdomains.IN. 5 NS v3a1.smartdomains.IN.
smartdomains.IN. 5 NS info349658.earth.orderbox-dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0