Apex domains cannot have CNAME records.
Question and response
Question
dig proton.ch. NS
Answer
proton.ch. 5 NS ns2.proton.ch.
proton.ch. 5 NS ns3.proton.ch.
proton.ch. 5 NS ns1.proton.ch.
Authority
The response has no authority records.
Additional
ns1.proton.ch. 5 A 185.70.42.150
ns2.proton.ch. 5 A 176.119.200.150
ns3.proton.ch. 5 A 205.132.47.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0