Apex domains cannot have CNAME records.
Question and response
Question
dig sys.kth.se. NS
Answer
sys.kth.se. 5 NS b.ns.kth.se.
sys.kth.se. 5 NS e.ns.kth.se.
sys.kth.se. 5 NS a.ns.kth.se.
Authority
The response has no authority records.
Additional
a.ns.kth.se. 3 A 130.237.72.246
b.ns.kth.se. 3 A 130.237.72.250
a.ns.kth.se. 3 AAAA 2001:6b0:1:0:0:0:0:246
b.ns.kth.se. 3 AAAA 2001:6b0:1:0:0:0:0:250
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0