Apex domains cannot have CNAME records.
Question and response
Question
dig at. NS
Answer
at. 5 NS j.ns.at.
at. 5 NS n.ns.at.
at. 5 NS r.ns.at.
at. 5 NS ns2.univie.ac.at.
at. 5 NS ns1.univie.ac.at.
at. 5 NS d.ns.at.
at. 5 NS u.ns.at.
Authority
The response has no authority records.
Additional
d.ns.at. 72732 A 81.91.161.98
j.ns.at. 72732 A 194.146.106.50
n.ns.at. 72732 A 81.91.173.130
r.ns.at. 72732 A 194.0.25.10
u.ns.at. 72732 A 185.102.12.2
ns1.univie.ac.at. 172681 A 78.104.144.2
ns2.univie.ac.at. 172681 A 192.92.125.2
d.ns.at. 72732 AAAA 2a02:568:20:1:0:0:0:d
j.ns.at. 72732 AAAA 2001:67c:1010:12:0:0:0:53
n.ns.at. 72732 AAAA 2a02:568:281:0:0:0:0:130
r.ns.at. 72732 AAAA 2001:678:20:0:0:0:0:10
u.ns.at. 72732 AAAA 2a02:850:ffff:0:0:0:0:2
ns1.univie.ac.at. 172681 AAAA 2001:628:2030:4301:0:0:0:2
ns2.univie.ac.at. 172681 AAAA 2001:678:1c:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0