Apex domains cannot have CNAME records.
Question and response
Question
dig 164.135.209.in-addr.arpa. NS
Answer
164.135.209.in-addr.arpa. 3 NS nsbos2.bostonfiber.net.
164.135.209.in-addr.arpa. 3 NS nsbos1.bostonfiber.net.
Authority
The response has no authority records.
Additional
nsbos1.bostonfiber.net. 3 A 23.157.24.157
nsbos2.bostonfiber.net. 3 A 23.157.24.158
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0