Apex domains cannot have CNAME records.
Question and response
Question
dig wien.at. NS
Answer
wien.at. 5 NS ns2.wien.gv.at.
wien.at. 5 NS ns1.wien.at.
wien.at. 5 NS ns11.govix.at.
wien.at. 5 NS ns5.univie.ac.at.
Authority
The response has no authority records.
Additional
ns1.wien.at. 5 A 217.149.233.230
ns5.univie.ac.at. 4 A 193.171.255.77
ns1.wien.at. 5 AAAA 2a00:1ba0:3:2503:0:0:0:230
ns5.univie.ac.at. 4 AAAA 2001:628:453:4305:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0