Apex domains cannot have CNAME records.
Question and response
Question
dig herokudns.com. NS
Answer
herokudns.com. 5 NS dns2.p05.nsone.net.
herokudns.com. 5 NS ns03.herokudns.net.
herokudns.com. 5 NS dns4.p05.nsone.net.
herokudns.com. 5 NS ns02.herokudns.net.
herokudns.com. 5 NS dns3.p05.nsone.net.
herokudns.com. 5 NS dns1.p05.nsone.net.
herokudns.com. 5 NS ns04.herokudns.net.
herokudns.com. 5 NS ns01.herokudns.net.
Authority
The response has no authority records.
Additional
ns01.herokudns.net. 5 A 45.54.92.1
ns02.herokudns.net. 5 A 45.54.92.65
ns03.herokudns.net. 5 A 45.54.92.129
ns04.herokudns.net. 5 A 45.54.92.193
ns01.herokudns.net. 5 AAAA 2607:f740:e651:0:0:0:0:1
ns02.herokudns.net. 5 AAAA 2607:f740:e651:4:0:0:0:1
ns03.herokudns.net. 5 AAAA 2607:f740:e651:8:0:0:0:1
ns04.herokudns.net. 5 AAAA 2607:f740:e651:c:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0