Apex domains cannot have CNAME records.
Question and response
Question
dig NIPR.MIL. NS
Answer
NIPR.MIL. 4 NS dns2.NIPR.MIL.
NIPR.MIL. 4 NS dns4.NIPR.MIL.
NIPR.MIL. 4 NS dns6.NIPR.MIL.
NIPR.MIL. 4 NS dns5.NIPR.MIL.
NIPR.MIL. 4 NS dns3.NIPR.MIL.
NIPR.MIL. 4 NS dns1.NIPR.MIL.
Authority
The response has no authority records.
Additional
DNS1.nipr.mil. 4 A 199.252.157.235
DNS2.nipr.mil. 4 A 199.252.162.243
DNS3.NIPR.MIL. 4 A 199.252.154.243
DNS4.nipr.mil. 4 A 199.252.143.243
DNS5.nipr.mil. 4 A 199.252.180.243
DNS6.NIPR.MIL. 4 A 199.252.155.243
DNS1.nipr.mil. 4 AAAA 2608:140:c:157:0:0:0:235
DNS2.nipr.mil. 4 AAAA 2608:120:c:162:0:0:0:243
DNS3.NIPR.MIL. 4 AAAA 2608:4122:2:154:0:0:0:243
DNS4.nipr.mil. 4 AAAA 2608:4163:1:143:0:0:0:243
DNS5.nipr.mil. 4 AAAA 2608:c184:1:180:0:0:0:243
DNS6.NIPR.MIL. 4 AAAA 2608:c144:1:155:0:0:0:243
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0