Apex domains cannot have CNAME records.
Question and response
Question
dig shell.eu. NS
Answer
shell.eu. 5 NS dns3.shell.com.
shell.eu. 5 NS dns4.shell.eu.
shell.eu. 5 NS dns1.shell.com.
shell.eu. 5 NS dns2.shell.com.
Authority
The response has no authority records.
Additional
dns1.shell.com. 5 A 134.146.80.20
dns2.shell.com. 5 A 144.199.196.20
dns3.shell.com. 5 A 134.163.128.20
dns4.shell.eu. 5 A 34.250.153.89
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0