Apex domains cannot have CNAME records.
Question and response
Question
dig ess.de.barracudanetworks.com. NS
Answer
ess.de.barracudanetworks.com. 5 NS ns-899.awsdns-48.net.
ess.de.barracudanetworks.com. 5 NS ns-1711.awsdns-21.co.uk.
ess.de.barracudanetworks.com. 5 NS ns-343.awsdns-42.com.
ess.de.barracudanetworks.com. 5 NS ns-1532.awsdns-63.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0