Apex domains cannot have CNAME records.
Question and response
Question
dig ns.tk. NS
Answer
ns.tk. 5 NS c.ns.tk.
ns.tk. 5 NS d.ns.tk.
ns.tk. 5 NS a.ns.tk.
ns.tk. 5 NS b.ns.tk.
Authority
The response has no authority records.
Additional
a.ns.tk. 155297 A 194.0.38.1
b.ns.tk. 155297 A 194.0.39.1
c.ns.tk. 155297 A 194.0.40.1
d.ns.tk. 155297 A 194.0.41.1
a.ns.tk. 155297 AAAA 2001:678:50:0:0:0:0:1
b.ns.tk. 155297 AAAA 2001:678:54:0:0:0:0:1
c.ns.tk. 155297 AAAA 2001:678:58:0:0:0:0:1
d.ns.tk. 155297 AAAA 2001:678:5c:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0