Apex domains cannot have CNAME records.
Question and response
Question
dig myfritz.net. NS
Answer
myfritz.net. 5 NS ns3.myfritz.net.
myfritz.net. 5 NS ns2.myfritz.net.
myfritz.net. 5 NS ns1.myfritz.net.
Authority
The response has no authority records.
Additional
ns1.myfritz.net. 5 A 217.110.95.230
ns2.myfritz.net. 5 A 212.42.244.13
ns3.myfritz.net. 5 A 212.42.224.76
ns1.myfritz.net. 5 AAAA 2001:920:197e:400:0:0:0:4
ns2.myfritz.net. 5 AAAA 2001:bf0:244:0:0:0:0:13
ns3.myfritz.net. 5 AAAA 2001:bf0:212:0:0:0:0:76
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0