Apex domains cannot have CNAME records.
Question and response
Question
dig grz.at. NS
Answer
grz.at. 5 NS dns4.grz.at.
grz.at. 5 NS dns3.grz.at.
grz.at. 5 NS dns1.grz.at.
grz.at. 5 NS dns2.grz.at.
Authority
The response has no authority records.
Additional
dns1.grz.at. 5 A 193.110.129.254
dns2.grz.at. 5 A 193.110.129.251
dns3.grz.at. 5 A 192.174.68.100
dns4.grz.at. 5 A 176.97.158.100
dns1.grz.at. 5 AAAA 2001:67c:2204:607:0:0:2ff0:0
dns2.grz.at. 5 AAAA 2001:67c:2204:607:0:0:2ff1:0
dns3.grz.at. 5 AAAA 2001:67c:1bc:0:0:0:0:100
dns4.grz.at. 5 AAAA 2001:67c:10b8:0:0:0:0:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0