Apex domains cannot have CNAME records.
Question and response
Question
dig DISA.mil. NS
Answer
DISA.mil. 5 NS dns4.DISA.mil.
DISA.mil. 5 NS dns2.DISA.mil.
DISA.mil. 5 NS dns1.DISA.mil.
DISA.mil. 5 NS dns3.DISA.mil.
DISA.mil. 5 NS dns5.DISA.mil.
Authority
The response has no authority records.
Additional
DNS1.DISA.MIL. 5 A 152.229.110.232
DNS2.DISA.MIL. 5 A 131.77.60.232
DNS3.DISA.MIL. 5 A 214.3.125.232
DNS4.DISA.MIL. 5 A 214.23.245.6
DNS5.DISA.MIL. 5 A 215.65.126.185
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0