Apex domains cannot have CNAME records.
Question and response
Question
dig alumni.cuhk.net. NS
Answer
alumni.cuhk.net. 5 NS dns2.cuhk.edu.hk.
alumni.cuhk.net. 5 NS dns3.cuhk.edu.hk.
alumni.cuhk.net. 5 NS dns1.cuhk.edu.hk.
Authority
The response has no authority records.
Additional
dns1.cuhk.edu.hk. 5 A 137.189.10.13
dns2.cuhk.edu.hk. 5 A 137.189.6.22
dns1.cuhk.edu.hk. 5 AAAA 2405:3000:3:118:0:0:0:10d
dns2.cuhk.edu.hk. 5 AAAA 2405:3000:3:6:0:0:0:16
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0