Apex domains cannot have CNAME records.
Question and response
Question
dig rev.booksys.com. NS
Answer
rev.booksys.com. 5 NS ns2.booksys.com.
rev.booksys.com. 5 NS ns3.booksys.com.
rev.booksys.com. 5 NS ns1.booksys.com.
Authority
The response has no authority records.
Additional
ns1.booksys.com. 5 A 206.126.96.179
ns2.booksys.com. 5 A 207.111.162.66
ns3.booksys.com. 5 A 66.128.61.70
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0