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