Apex domains cannot have CNAME records.
Question and response
Question
dig hkfree.org. NS
Answer
hkfree.org. 5 NS ns3.hkfree.org.
hkfree.org. 5 NS ns1.hkfree.org.
hkfree.org. 5 NS ns2.hkfree.org.
Authority
The response has no authority records.
Additional
ns1.hkfree.org. 5 A 89.248.240.1
ns2.hkfree.org. 5 A 89.248.240.3
ns3.hkfree.org. 5 A 37.205.11.42
ns1.hkfree.org. 5 AAAA 2a01:168:4:0:0:0:0:100
ns2.hkfree.org. 5 AAAA 2a01:168:4:0:0:0:0:129
ns3.hkfree.org. 5 AAAA 2a03:3b40:100:0:0:0:1:156
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0