Apex domains cannot have CNAME records.
Question and response
Question
dig topdns.com. NS
Answer
topdns.com. 5 NS ns-uk.topdns.com.
topdns.com. 5 NS ns-usa.topdns.com.
topdns.com. 5 NS ns-canada.topdns.com.
Authority
The response has no authority records.
Additional
ns-uk.topdns.com. 5 A 77.247.183.137
ns-usa.topdns.com. 5 A 46.166.189.99
ns-usa.topdns.com. 5 A 161.35.225.48
ns-canada.topdns.com. 5 A 109.201.142.225
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0