Apex domains cannot have CNAME records.
Question and response
Question
dig otrs.stackdata.net. NS
Answer
otrs.stackdata.net. 1 NS ns-01.stackdata.net.
otrs.stackdata.net. 1 NS ns-02.stackdata.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0