Apex domains cannot have CNAME records.
Question and response
Question
dig muc.internetx.de. NS
Answer
muc.internetx.de. 5 NS nsd.domain-robot.org.
muc.internetx.de. 5 NS nsa.domain-robot.org.
muc.internetx.de. 5 NS nsb.domain-robot.org.
muc.internetx.de. 5 NS nsc.domain-robot.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0