Apex domains cannot have CNAME records.
Question and response
Question
dig ip-91-134-40.eu. NS
Answer
ip-91-134-40.eu. 4 NS ns16.ovh.net.
ip-91-134-40.eu. 4 NS dns16.ovh.net.
Authority
The response has no authority records.
Additional
ns16.ovh.net. 2 A 5.39.116.25
dns16.ovh.net. 2 A 5.135.112.57
ns16.ovh.net. 2 AAAA 2001:41d0:b00:ef00:0:0:0:2
dns16.ovh.net. 2 AAAA 2001:41d0:d00:f500:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0