Apex domains cannot have CNAME records.
Question and response
Question
dig static.ef-service.nl. NS
Answer
static.ef-service.nl. 5 NS dns2.ef-service.nl.
static.ef-service.nl. 5 NS dns1.ef-service.nl.
Authority
The response has no authority records.
Additional
dns1.ef-service.nl. 5 A 89.20.160.230
dns2.ef-service.nl. 5 A 195.216.224.5
dns1.ef-service.nl. 5 AAAA 2a02:fe9:0:0:0:0:0:8
dns2.ef-service.nl. 5 AAAA 2001:67c:294:0:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0