Apex domains cannot have CNAME records.
Question and response
Question
dig zdnscloud.net. NS
Answer
zdnscloud.net. 4 NS cns1.zdnscloud.net.
zdnscloud.net. 4 NS vns1.zdnscloud.biz.
zdnscloud.net. 4 NS fns1.zdnscloud.cn.
zdnscloud.net. 4 NS dns1.zdnscloud.info.
zdnscloud.net. 4 NS ins1.zdnscloud.com.
Authority
The response has no authority records.
Additional
cns1.zdnscloud.net. 4 A 42.62.2.24
cns1.zdnscloud.net. 4 A 42.62.2.29
dns1.zdnscloud.info. 4 A 14.29.124.15
dns1.zdnscloud.info. 4 A 14.116.247.213
fns1.zdnscloud.cn. 5 A 42.62.2.24
fns1.zdnscloud.cn. 5 A 42.62.2.29
ins1.zdnscloud.com. 4 A 27.221.63.3
ins1.zdnscloud.com. 4 A 119.167.244.44
vns1.zdnscloud.biz. 4 A 203.99.22.3
vns1.zdnscloud.biz. 4 A 203.99.23.3
cns1.zdnscloud.net. 4 AAAA 2408:8719:2500:0:a:0:0:1
dns1.zdnscloud.info. 4 AAAA 2408:8719:2500:0:b:0:0:1
fns1.zdnscloud.cn. 5 AAAA 2408:8719:2500:0:a:0:0:1
ins1.zdnscloud.com. 4 AAAA 2401:8d00:f:0:0:0:0:1
vns1.zdnscloud.biz. 4 AAAA 2401:8d00:9:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0