Apex domains cannot have CNAME records.
Question and response
Question
dig gslb.verisign.com. NS
Answer
gslb.verisign.com. 0 NS gslb.ash3.verisign.com.
gslb.verisign.com. 0 NS gslb.brn1.verisign.com.
gslb.verisign.com. 0 NS gslb.ilg1.verisign.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0