Apex domains cannot have CNAME records.
Question and response
Question
dig u-3mrs.fr. NS
Answer
u-3mrs.fr. 5 NS ns1.univmed.fr.
u-3mrs.fr. 5 NS cnudns.cines.fr.
Authority
The response has no authority records.
Additional
ns1.univmed.fr. 5 A 139.124.1.1
cnudns.cines.fr. 5 A 193.48.169.40
ns1.univmed.fr. 5 AAAA 2001:660:5402:801:0:0:0:1
cnudns.cines.fr. 5 AAAA 2001:660:6301:301:0:0:2:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0