Apex domains cannot have CNAME records.
Question and response
Question
dig wu.ac.at. NS
Answer
wu.ac.at. 5 NS ns1.wu-wien.ac.at.
wu.ac.at. 5 NS ns10.univie.ac.at.
wu.ac.at. 5 NS ns5.univie.ac.at.
wu.ac.at. 5 NS ns2.wu-wien.ac.at.
Authority
The response has no authority records.
Additional
ns1.wu-wien.ac.at. 5 A 137.208.10.10
ns2.wu-wien.ac.at. 5 A 137.208.20.10
ns5.univie.ac.at. 5 A 193.171.255.77
ns10.univie.ac.at. 5 A 192.76.243.2
ns5.univie.ac.at. 5 AAAA 2001:628:453:4305:0:0:0:53
ns10.univie.ac.at. 5 AAAA 2001:67c:133c:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0